Overbore. Defined. Denied.

[corbanzo]

So I haven't opened a can of worms in a while... and I hate the word overbore... so here it goes.  This term is rampant with big cased medium bore rifles. 


There are no "overbore" cartridges in existence. 


The word overbore by definition means overcome by physical force.  This reference in firearms usually refers to giant cases and small bullets.  Cue the 7mm ultra mag which I have some years of experience with, the .50 BMG which of course has more powder than them all, and rounds like the small 223 super short mags and such. 


So why do peoples perception of these big cases and small bullets make them somehow think that there is something intrinsically wrong with the cartridge just because it is slightly more inefficient?  When in reality any gun out there that is running less than 60,000psi could be considered less than efficient by a modern standard.  There seems to be some belief that they aren't burning all their powder, or we are shooting this bullet out of a barrel that is way too short, or any number of excuses to discount the fact that a big cases makes a whoooole bunch of pressure and make bullets shoot rreeeeeeeaaalllyy fast!  Which is good in my mind.


The reality of it stands that powder is burnt in the first couple of inches.  So unburnt powder is just right out.  Some powders do leave residue, but that isnt unburnt.  Compression under the bullet makes so much heat when that round goes off, there is no way for the combustibles to last more than half a nanosecond. 


Then as the bullet accelerates - wouldn't you want as much pressure as humanly possible?  I want that pressure curve to shoot right up to sammi spec!  And i want it to stay there!  And the bigger the case, the more in^2 for the pressure to build on, the more total pressure, the less drop in pressure down the barrel.  So what if you bullet exists the barrel and you are still in a high part of the power curve?  Does that mean anything?  Do you get more muzzle blast?  Yes.  Do you get more recoil?  Yes.  Do you get a bigger fireball?  Hell yes!  And what is wrong with all of these things?  Nothing.  If there was a problem with them we would all be shooting .22's for fun and everybody would hunt with bow and arrow for deer (not saying there is anything wrong with that)    Why do you have to try and make a level at which there is "too much" left over pressure?  There has to be left over pressure.  You aren't going to use it up.  If you are at the 0 mark when your bullets leaves the barrel... you got some problems!  Every gun has muzzle blast.  It's what they are made for!  BOOM!  Higher average pressure = more acceleration.  Simple. 


I just get annoyed when people say my 7RUM is "overbore" or my .458 lott is "overkill" because I'm up at 60% on the power scale when my bullet exits the barrel and your 7mag or 45-70 is down at 20%.  (not scientific numbers, FYI)  I'm glad you are using less powder and all of that... but I care about POWER.  Raw speed and energy. I don't care how I get there.  I'm an big American meat eating capitalist! 


Is the speed harder on barrels and ears and your face?  Yeah, it is.  But there is nothing about which goes to the point where anything is actually physically overcome by the force of it. 


If anyone can show me a cartridge in which the pressure scale goes to and stays at 100% of sammi maximum before it exits the barrel, then I will agree that there is something that is actually "overbore" and cannot gain in velocity.  But this doesn't exist, and you would have to have such a ridiculously slow burning powder to do this in the first place.  I agree that there are diminishing returns with such large case capacities, as pressures can only get so high, but there is still a gain along the pressure curve as it drops more slowly.  If you did take the same case or a bigger case, and increased the maximum pressure rating, what is the result then??  Higher acceleration and...  More velocity!!!  Yay!!



So yes, certain cases are less efficient than others.  But as long as there can be increased pressure ratings, change in barrel twist, and good bullet design, there is no case - and will be - no case of which their powder capacity outpaces the physical limitations of the firearm and bullet. 

[Lloyd Smale]

I agree totaly. I never fail to get a chuckle at guys who will claim the 264 mag is grossly overbore and a terrible barrel burner but will sing the praises of the 7mag which is about identical. When I find a round that does worse with more powder ill eat crow but until then ill call bs on it too. From all ive seen a 7mag will leave a 280 in its dust and a 300 wby will leave a 3006 or even a 300 win mag in its dust. then you have the guys who sing the praises of the short mags. Claim there majic because they will run right with a standard mag and in some factory offerings will. When you cut to the chase youll see the ONLY reason they do it is they run pressures so high there flirting with flattened primers with factory ammo. Load a standard mag to the same pressure and again it will leave the short mag in the dust.

[nomosendero]

Bigger cartridge, faster. Well yes.
 
Yes, I agree with both of you.
The term overbore is a severely abused term. The folks that fuss the most about these things don't shoot enough to wear out the bore of a 7Rum.
 
No one can argue that we can get the same barrel life from a 300RUM that you will get from a 308, but that is far different from Corbanzo's point that you can gain more PERFORMANCE from the bigger case. The last paragraph is important in that you can gain more performance
from larger cases by increasing the twist rates to allow for heavier bullets, longer tubes, different powders, etc. The 7RUM is a good example. With a factory tube for it and a 7STW, you gain very little with the RUM, but you can shoot a bullet like the new Berger Hybrid in a custom fast twist and gain a lttle more with the RUM. I like the overall tradeoff of the 7STW or 7 Rogue, but it depends on your goals.
 
As Lloyd said, there is a bunch of hype with the Short Mags & some of the gun companies still tout this untruth. I watched a Ruger show touting their even smaller mags & hearing Hornady say that that the little 300 runs right up there with the Win mag, never telling folks they have the little round loaded to higher pressures. They don't even come close when loaded to equal pressures.
 
Diminishing returns occurs with everthing, but you still get returns, so as always it boils down to your goals you want to achieve with that particular rifle.
 
"Overbore" gets the same amount of attention for me as the term "Overkill" & used by pretty much the same people.

[roper]

So I haven't opened a can of worms in a while... and I hate the word overbore... so here it goes.  This term is rampant with big cased medium bore rifles. 


There are no "overbore" cartridges in existence. 


The word overbore by definition means overcome by physical force.  This reference in firearms usually refers to giant cases and small bullets.  Cue the 7mm ultra mag which I have some years of experience with, the .50 BMG which of course has more powder than them all, and rounds like the small 223 super short mags and such. 


So why do peoples perception of these big cases and small bullets make them somehow think that there is something intrinsically wrong with the cartridge just because it is slightly more inefficient?  When in reality any gun out there that is running less than 60,000psi could be considered less than efficient by a modern standard.  There seems to be some belief that they aren't burning all their powder, or we are shooting this bullet out of a barrel that is way too short, or any number of excuses to discount the fact that a big cases makes a whoooole bunch of pressure and make bullets shoot rreeeeeeeaaalllyy fast!  Which is good in my mind.


The reality of it stands that powder is burnt in the first couple of inches.  So unburnt powder is just right out.  Some powders do leave residue, but that isnt unburnt.  Compression under the bullet makes so much heat when that round goes off, there is no way for the combustibles to last more than half a nanosecond. 


Then as the bullet accelerates - wouldn't you want as much pressure as humanly possible?  I want that pressure curve to shoot right up to sammi spec!  And i want it to stay there!  And the bigger the case, the more in^2 for the pressure to build on, the more total pressure, the less drop in pressure down the barrel.  So what if you bullet exists the barrel and you are still in a high part of the power curve?  Does that mean anything?  Do you get more muzzle blast?  Yes.  Do you get more recoil?  Yes.  Do you get a bigger fireball?  Hell yes!  And what is wrong with all of these things?  Nothing.  If there was a problem with them we would all be shooting .22's for fun and everybody would hunt with bow and arrow for deer (not saying there is anything wrong with that)    Why do you have to try and make a level at which there is "too much" left over pressure?  There has to be left over pressure.  You aren't going to use it up.  If you are at the 0 mark when your bullets leaves the barrel... you got some problems!  Every gun has muzzle blast.  It's what they are made for!  BOOM!  Higher average pressure = more acceleration.  Simple. 


I just get annoyed when people say my 7RUM is "overbore" or my .458 lott is "overkill" because I'm up at 60% on the power scale when my bullet exits the barrel and your 7mag or 45-70 is down at 20%.  (not scientific numbers, FYI)  I'm glad you are using less powder and all of that... but I care about POWER.  Raw speed and energy. I don't care how I get there.  I'm an big American meat eating capitalist! 


Is the speed harder on barrels and ears and your face?  Yeah, it is.  But there is nothing about which goes to the point where anything is actually physically overcome by the force of it. 


If anyone can show me a cartridge in which the pressure scale goes to and stays at 100% of sammi maximum before it exits the barrel, then I will agree that there is something that is actually "overbore" and cannot gain in velocity.  But this doesn't exist, and you would have to have such a ridiculously slow burning powder to do this in the first place.  I agree that there are diminishing returns with such large case capacities, as pressures can only get so high, but there is still a gain along the pressure curve as it drops more slowly.  If you did take the same case or a bigger case, and increased the maximum pressure rating, what is the result then??  Higher acceleration and...  More velocity!!!  Yay!!



So yes, certain cases are less efficient than others.  But as long as there can be increased pressure ratings, change in barrel twist, and good bullet design, there is no case - and will be - no case of which their powder capacity outpaces the physical limitations of the firearm and bullet.

Are you talking about a custom barrel or factory rifle?  You can change barrel twist and length on a 30-06 and get an increase over published data and that's no big deal. 

As to the WSM hype it's been around long time and lot of internet experts tell you everything bad about one but for some unknown reason there still around. 

[nomosendero]

Unknown to whom? Marketing works if done well.
 
The others are still around too.

[Lloyd Smale]

lots more effects barrel life then the size of the case. Probably the three major factors are type and burning speed of the powder and pressure loaded to. The old 264 had a bad rep. Why? because back then it was loaded with winchester ball powder by winchester and 4350 by everyone else because thats what they had. remingtons 7mag had a slightly better rep because at least they used 4350 in it. today we have a vast array of slower burning stick powders that get the job done much better. Id bet my house that if you took two identical rifles. One in 300wsm and one in 300 win mag and one in 300wby. Loaded them all to the SAME VELOCITY that the short mags barrel would go first. The 300wby would take a bit more powder but would be idling in pressure to get the same velocity the 300wsm that needed 60k of pressure to get there did. Now load the 300wby up to the same pressure as the wsm and no doubt its going to burn a barrel faster. Like was said in another post the average guy will never burn a barrel. It isnt like your out in a dog town shooting 500 rounds a day out of a 7rem ultra mag. Most guys, including myself, with a gun like that put maybe 2 or 300 rounds through one doing load developement until they find a load or two they like and then shoot big game with them. I shoot more deer then probably anyone here and couldnt imagine even me burning out a barrel in a rifle used for deer hunting. Even if you shot say 20 rounds out of the gun before season your going to have a barrel that will be still good to shoot 500-1000 deer and boys thats alot of deer. I shoot between 50 and a 100 deer a year and have yet to have to replace a rifle barrel. As a matter of fact youd play hell finding a rifle in my safe that even shows the first signs of barrrel wear that is a big game hunting gun. Realisticaly not many guys actually buy a 257wby, 264 mag, 7mag, 300 mag or a 7stw or ultra mag to target practice with or shoot prarie dogs. Mostly its a bogus argument that guys that cant handle the recoil of a big gun use to justify the fact that there not willing to put in the time to master one. If you hunt where long shots are rare and dont need a big gun or have physical limitations that dont allow you too or if you just dont want to put the time in so be it. But it ###### me off that guys feel a need to bad mouth them or make up stupid reasons to justify not using them or make out guys who do to be some kind of macho gorilla.  To me its like driving a car with 400 hp. Sure you dont need it everyday but its nice to have when you do and it sure is more fun then a prius!!

[roper]

lots more effects barrel life then the size of the case. Probably the three major factors are type and burning speed of the powder and pressure loaded to. The old 264 had a bad rep. Why? because back then it was loaded with winchester ball powder by winchester and 4350 by everyone else because thats what they had. remingtons 7mag had a slightly better rep because at least they used 4350 in it. today we have a vast array of slower burning stick powders that get the job done much better. Id bet my house that if you took two identical rifles. One in 300wsm and one in 300 win mag and one in 300wby. Loaded them all to the SAME VELOCITY that the short mags barrel would go first. The 300wby would take a bit more powder but would be idling in pressure to get the same velocity the 300wsm that needed 60k of pressure to get there did. Now load the 300wby up to the same pressure as the wsm and no doubt its going to burn a barrel faster. Like was said in another post the average guy will never burn a barrel. It isnt like your out in a dog town shooting 500 rounds a day out of a 7rem ultra mag. Most guys, including myself, with a gun like that put maybe 2 or 300 rounds through one doing load developement until they find a load or two they like and then shoot big game with them. I shoot more deer then probably anyone here and couldnt imagine even me burning out a barrel in a rifle used for deer hunting. Even if you shot say 20 rounds out of the gun before season your going to have a barrel that will be still good to shoot 500-1000 deer and boys thats alot of deer. I shoot between 50 and a 100 deer a year and have yet to have to replace a rifle barrel. As a matter of fact youd play hell finding a rifle in my safe that even shows the first signs of barrrel wear that is a big game hunting gun. Realisticaly not many guys actually buy a 257wby, 264 mag, 7mag, 300 mag or a 7stw or ultra mag to target practice with or shoot prarie dogs. Mostly its a bogus argument that guys that cant handle the recoil of a big gun use to justify the fact that there not willing to put in the time to master one. If you hunt where long shots are rare and dont need a big gun or have physical limitations that dont allow you too or if you just dont want to put the time in so be it. But it ###### me off that guys feel a need to bad mouth them or make up stupid reasons to justify not using them or make out guys who do to be some kind of macho gorilla.  To me its like driving a car with 400 hp. Sure you dont need it everyday but its nice to have when you do and it sure is more fun then a prius!!

   


Here is one of your post about the 300WSM on another site

Not about reading it somewhere also. Been shooting the WSM since it came out and built probably the first comp gun with it, so 10 years of experience with it AND the Win Mag, 6 WSM guns and numerous barrels I have a real good handle on what is and isn't with both.[/QUOTE]



Here is late post about the 300WSM same site

 Personaly i think the short mags were an answer to a question that didnt exist. Like I said i have nothing against the 300wsm. If a great deal came up on one i might even buy one but i surely am not buying one figurig its going to do a pinch of *** more then my 300 win mags allready do.

Why don't you try ranting over there and try place bet you might wind up homeless.

[Lloyd Smale]

i dont understand the jist of your post. If you like the short mags so be it. I never once said they didnt work. Just that the advantages to them are blown out of porportion. Im surely not badmouthing magnums as i use them about everyday.
this statement sure isnt mine==
Not about reading it somewhere also. Been shooting the WSM since it came out and built probably the first comp gun with it, so 10 years of experience with it AND the Win Mag, 6 WSM guns and numerous barrels I have a real good handle on what is and isn't with both.[/QUOTE]
 
the second quote you posted --
 Personaly i think the short mags were an answer to a question that didnt exist. Like I said i have nothing against the 300wsm. If a great deal came up on one i might even buy one but i surely am not buying one figurig its going to do a pinch of *** more then my 300 win mags allready do.
 
Is mine and i stand behind it. Id really like to know one REAL world advantage they have over the standard mags. Other then maybe saving a bit of powder which doesnt mean much to me. Maybe as a comp gun they work, i dont know. I dont shoot comp i shoot deer and id about bet thats the use 99 percent of the readers here are interested in.
 
If you are in disagreement with my thoughts how about posting your own. If you like them buy them. I never once said my opinion was the only one and if everyone liked the same thing as me it would be a pretty boring world.

[corbanzo]

First one says he has experience with the 6mil WSM, second one says one day he might buy a .300 WSM.  Makes sense to me.


I have years of experience with the 7mm RUM, and I just this year bought a .338 RUM.  Been talking about buying one of those for a while.  Whoopidy.


Roper if your plans on posting are to try and use other people's tongues against them, I would recommend you do it elsewhere. 


And as for the short mags and performance... performance is all about case capacity.  The square and cubic inches inside the chamber.  The rounder an object, the more its surface area grows in respect to its volume.  Mostly the short mags were about getting the same amount of powder in a shorter case which shortens your bolt throw, as to their "efficiency".... who knows.  The powder does burn closer to the barrel in a shorted case... so may there be some shortening of barrel life and throat cutting?  It is possible.  But any case isn't intrinsically a barrel burner.  Speed and pressure are what do most of it - and you can get that in any round. 


I don't think short mags are hype at all.  Easier to carry, shorted bolt throw, same power, and they are modern design - meaning the guns and the brass are designed for higher pressure.  Designed for performance, nothing wrong with that at all. 


And if you are taking the time to look up lloyd's posts on other forums... you surely must be impressed with the depth of his knowledge.


I gotta tell you guys though... I have been eyeing that .30 OSSM!  Pretty much a -06AI in an AR-15 platform?  Yes please.  Sorry wolves. 

[roper]

i dont understand the jist of your post. If you like the short mags so be it. I never once said they didnt work. Just that the advantages to them are blown out of porportion. Im surely not badmouthing magnums as i use them about everyday.
this statement sure isnt mine==
Not about reading it somewhere also. Been shooting the WSM since it came out and built probably the first comp gun with it, so 10 years of experience with it AND the Win Mag, 6 WSM guns and numerous barrels I have a real good handle on what is and isn't with both.

 
the second quote you posted --
 Personaly i think the short mags were an answer to a question that didnt exist. Like I said i have nothing against the 300wsm. If a great deal came up on one i might even buy one but i surely am not buying one figurig its going to do a pinch of *** more then my 300 win mags allready do.
 
Is mine and i stand behind it. Id really like to know one REAL world advantage they have over the standard mags. Other then maybe saving a bit of powder which doesnt mean much to me. Maybe as a comp gun they work, i dont know. I dont shoot comp i shoot deer and id about bet thats the use 99 percent of the readers here are interested in.
 
If you are in disagreement with my thoughts how about posting your own. If you like them buy them. I never once said my opinion was the only one and if everyone liked the same thing as me it would be a pretty boring world.
[/quote] 


Lloyd I don't buy anything as I have things build and I do post on LRH site been a member a number of years over 10.  I stay out of the post that deal with something like 300mag vs 300WSM on which you posted as they just get into a p****** contest vs talking about the good points.  It's the same here.

I've never care one bit about barrel wear so how long a barrel last means nothing to me as I can afford to replace  barrels.  Why would anyone shoot a rifle they have to worry about rd count or if the rifles overbore etc.  I shoot couple custom 300WSM my latest one I did post about it on LRH and so I pretty much deal with guy's who have rifles build over there. 

[Lloyd Smale]

I too dont worry about barrel life as like i said i dont shoot a deer hunting gun enough to wear one out. My bolt guns all get used so its rare that any one of them gets shot 20 times in a season. So other then load development there not shot enough to wear in my lifetime. It all comes down to what floats your boat. Some will chuckle at the doe i shot tonight with the stw. Bit of an overkill? Maybe so but theres backstraps going in the freezer tomarrow. Like i posted i sure wouldnt be beyond buying a short mag someday myself for something new to play with. Knowing though that theres basicaly no advantage over what i allready have. But life gets boring if you hunt with the same thing all the time. Guys got to get AT LEAST one new rifle every year to play with.

[D Fischer]

Over bore or over kill, what difference does either make? I don't care for the over bore cartridges for any reason other than they recoil more than a cartridge that is not. But then recoil is subjective too. I used to shoot a 338 win mag so much that I could put over a box of ammo through it and not be bothered with the recoil. Then there's my pet 25-06, I suspect that it is over bore capacity, don't care. Recoil is very tolerable and I'm not going to shoot out the barre out of it in my lifetime. Might consider the view of barrel life. just what does that really mean? A friend many years ago used a 264 win mag a lot. Very accurate rifle and he kept it loaded hot. At about 1500 rds he told me the barrel was burned out as it wasn't accurate any more. The thing still shot close to one inch with good hand loads where before it was under down close to 3/4". A lot of things could account for that little loss of accuracy other than barrel burnout.

If I were into shooting over bore magnums, which would be about any magnum and several small bore's on standard cases, it would not bother me that people talked about it. sure it's over bore, so what? Sure it's probably less efficient too, so what? The good part is they do not have to buy the powder, do not have to shoot it and if the barrel really does burn out, they don't have to replace the barrel. I have no idea why guy's shoot big magnums and I used to shoot them myself. Don't care either if it's good with them, it's good with me.

One thing that does bother me is some of the people shooting hard recoilers that it doesn't bother step right up and recommend one of them to a new shooter. Well maybe they can handle it and maybe they can't. No problem if they can but what if they can't? Then they are suggested all kinds of recoil reducer's and all they want to do is shoot a deer! I think that is stupid and little more than people that love their big magnum's wanting to let the world know how tuff they really are. Otherwise, I could care less what someone want's to shoot.

[Lloyd Smale]

Dfisher makes a good point. More important then the fact they burn a bit more powder or have a couple hundred round shorter barrel life is if the man behind them can REALLY handle them. I lost count of the people ive seen claim to be able to handle a big gun and probably about a 1/10 of them really can. If your not willing to put in the time to master one your sure much better off with a smaller gun. I wonder how many animals die a miseable death because of egos.

[RevGeo]

Sometimes I don't get the worry over recoil. In a big game hunting rifle the only time recoil would be a consideration is while sighting in on a bench, and then padding can be used to protect the shoulder and as long as the gun is held correctly the shooter won't get whacked in the eyebrow by the scope.
When shooting a game animal recoil and muzzle blast are hardly noticed, at least by me. It's not like you're going to pop off a couple of hundred rounds of .338 at a prairie dog town (though that might be fun..) during the course of an afternoon. Let's face it, you're lucky to shoot the gun once during big game season.
I've shot a .458 a few times (never from a bench) and while it wasn't fun, it didn't leave any lasting damage that I'm aware of. Personally I can tell little difference between the 30-06, 7mm Mag, .35 Whelen and the various .300 Mags recoil-wise. The .338 and .375 feel like shooting a 12 ga. with slugs to me. Maybe it's because I grew up shooting with my father - it was just a given that a rifle kicks when you shoot it. It's part of shooting one. And they go 'boom' pretty loudly as well.
Staying on topic, the same thing applies to 'overbore' cartridges. If a dedicated varmint shooter (which I am not) uses a .257 Weatherby for example, he might burn his barrel in 500-750 rounds, which is a lot of varmint hunting. So what? A race car hobbyist will probably be changing or rebuilding an entire engine yearly. Overbore big game rifles probably aren't shot enough to worry about barrel burnout anyway, unless the shooter just likes to burn powder or target shoots a lot.
Practicallity be damned - this is supposed to be fun!

[Lloyd Smale]

Practicallity be damned - this is supposed to be fun!

amen to that!

[corbanzo]

Some people are made for it and some people aren't i guess!  I have a few .22's and they have really become safe queens.  People talk about shooting all day with their .22's...  I'd rather shoot for a couple hours with some boomers!  That's what I call efficiency.  More fun, less time.   


Even the people who don't want to step above their .223's I'm sure are still on youtube searching for .577 tyrannosaur...  You might not wanna do it, but don't knock us for doing it! 


And you can laugh at our scope marks and shoulder bruises and scoff at what we pay and how much powder we burn... and be jealous of our big fat %*&^ eating grins!!!

[mrbigtexan]

im liking this thread! my biggest rifle i have is a 7mag, but i always wonder how i would like the stw or the rum because they interest me, but i dont personnally know anyone who has one.   If i tried one out first then i might bite off on one. I also have a 7mm saum and i really like the model 7 that it is, but it s really no different then the 7mag and i wonder how long its gonna be available. But, if i end up keeping it there will always be options if i ever do ever really wear it out, which i doubt.

[Lloyd Smale]

I cant say much about the stws recoil. Mine was bought used and it has a factory muzzle break on it and ive never shot it without it. Ill say this though. With it the recoil is almost scary but in a good way. It kicks about like an 8lb 243. It sure does make some noise though!!!

[helotaxi]

And the bigger the case, the more in^2 for the pressure to build on, the more total pressure, the less drop in pressure down the barrel.

That's not how it works.  Pressure exerted on the case does nothing but apply pressure to the chamber walls and bolt face.  Does nothing to accelerate the bullet.  Only pressure on the base of the bullet does that.  That's why for a given pressure curve, barrel length and bullet weight, the bullet of the larger caliber has the highest velocity.  A big case holds more powder.  More powder can produce more gas.  More gas sustains higher pressure longer as the bullet accelerates down the barrel.  And actually with the super slow powders being burned in the mega magnums, significant amounts of powder are still burning when the bullet is half-way down the barrel.  That's kinda the point of having a huge case of slow burning powder in the first place.  If the powder all burned up in the first couple inches of barrel, you couldn't get a sustained pressure curve.

So what if you bullet exists the barrel and you are still in a high part of the power curve?  Does that mean anything?  Do you get more muzzle blast?  Yes.  Do you get more recoil?  Yes.  Do you get a bigger fireball?  Hell yes!  And what is wrong with all of these things?

In that scenario, you'd get more of what you really want, velocity, with a longer barrel.  "Overbore" cartridges see bigger differences in velocity with changes in barrel length.  That is the only "issue" if you will with huge cases and relatively small bores.  Nothing wrong with sacrificing efficiency for the utmost in velocity, but if you're going that route, mating such a cartridge to a short barrel is a waste.  It's not like you need a super light weight and handy rifle in 7 RUM.  You use such a cartridge to shoot things at very long range.  You're only going to fire a shot or two so a heavy barrel contour isn't needed so why not go with the 26+" tube?  The lighter contour barrel isn't going to have an effect on accuracy and isn't that much heavier.  You're burning the powder and sucking up the recoil anyway, why not get what you're paying for?  You can come really close to RUM velocity from a short barrel with smaller cartridges with less blast and less recoil.

If you get right down to it, "overbore" is only relevant or descriptive with a certain length barrel, assuming that you have a slow enough powder available to take advantage of the case capacity while remaining within pressure limits in the first place.  If you can't fill the case with US869 or 20N29 without exceeding pressure limits, you have wasted case capacity and are truly overbore regardless of barrel length.  For example, if you have 22" of barrel to play with a 7mm RSAUM makes a lot of sense.  The cartridge was essentially designed around that barrel length.  Yes, you'd get more velocity with a longer barrel, but not a significant amount.  On the other hand, putting a 7mm Rem Mag in that same 22" barrel wastes a lot of potential and might leave you short of the SAUM in velocity from that barrel length, even though the RM has more case capacity.

[Dave in WV]

Instead of using the word "overbore" I've read some writers use efficient or inefficient.

[nomosendero]

Instead of using the word "overbore" I've read some writers use efficient or inefficient.

And the 30 Carbine round is more "efficient" than a 308. So we should find the balance of performance WE want and run with it.

[rosewood]

I thought overbore was when you drill out the cylinder of an engine to a larger diameter.  Ergo, the bore is now over sized to the original bore.  In the case of firearms, I would think that would be when you re-drilled the barrel to a larger caliber.  What the heck does "overbore" have to do with case size or amount of gunpowder?  Beats me.   I'm just sayin'.   

[quatroclick]

Magnum, overbore, or just "big for caliber" cartridges exist for the same reasons that small block and big block corvettes existed.  Different people want different experiences.  Were big block corvettes over represented in "car wrapped around a tree" statistics?  Probabaly. 


I read a very interesting article a few years ago written by a hunting guide about the calibers most likely to result in wounded animals.  I don't remember the exact numbers (he kept good notes), but the overwhelming majority of the wounded animals were shot with magnum calibers.  The point was not that magnums were inferior.  Proper bullet placement combined with proper bullet choice for the velocity at impact will result in reliably dead animals.  Magnums, whether overbore or not for the most part, place a premium on choosing the right bullet.  More importantly, as recoil and muzzle blast goes up, the pool of people able to accurately place the bullet on target drops at an exponential rate.  An awful lot of hunters who should be carrying a .243 or .257 Roberts are packing 300 Weatherbys etc.

[Lloyd Smale]

well were done crop damage shooting for the year. Last two deer were shot last night with a 300 wby using 180 barnes tsx bullets. Both were shot between 300 and 350 yards and neither had any more meat damage then i would have gotten with a 3006. One shot through the ribs, dropped in her tracks with no meat damage. the other shot right through both front shoulders and it ruined most of those shoulders which would have happened with any 270 or 06 with the same bullet placement. Was it needed? NOPE. Did it work well? YUP. Anytime i shoot two deer at one time and dont have to take one step to track them i consider a success. Bottom line is most who badmouth them have never used them or just dont want or cant handle the power level. that big block chev will cruise down the road at 60mph just like the small block one will. Sure it may use a bit more fuel to do it but in the world of reloading that extra 20 grains of powder never caused anyone to be so broke they couldnt afford to pay there bills.

[corbanzo]

And the bigger the case, the more in^2 for the pressure to build on, the more total pressure, the less drop in pressure down the barrel.

That's not how it works.  Pressure exerted on the case does nothing but apply pressure to the chamber walls and bolt face.  Does nothing to accelerate the bullet.  Only pressure on the base of the bullet does that.  That's why for a given pressure curve, barrel length and bullet weight, the bullet of the larger caliber has the highest velocity.  A big case holds more powder.  More powder can produce more gas.  More gas sustains higher pressure longer as the bullet accelerates down the barrel.  And actually with the super slow powders being burned in the mega magnums, significant amounts of powder are still burning when the bullet is half-way down the barrel.  That's kinda the point of having a huge case of slow burning powder in the first place.  If the powder all burned up in the first couple inches of barrel, you couldn't get a sustained pressure curve.

So what if you bullet exists the barrel and you are still in a high part of the power curve?  Does that mean anything?  Do you get more muzzle blast?  Yes.  Do you get more recoil?  Yes.  Do you get a bigger fireball?  Hell yes!  And what is wrong with all of these things?

In that scenario, you'd get more of what you really want, velocity, with a longer barrel.  "Overbore" cartridges see bigger differences in velocity with changes in barrel length.  That is the only "issue" if you will with huge cases and relatively small bores.  Nothing wrong with sacrificing efficiency for the utmost in velocity, but if you're going that route, mating such a cartridge to a short barrel is a waste.  It's not like you need a super light weight and handy rifle in 7 RUM.  You use such a cartridge to shoot things at very long range.  You're only going to fire a shot or two so a heavy barrel contour isn't needed so why not go with the 26+" tube?  The lighter contour barrel isn't going to have an effect on accuracy and isn't that much heavier.  You're burning the powder and sucking up the recoil anyway, why not get what you're paying for?  You can come really close to RUM velocity from a short barrel with smaller cartridges with less blast and less recoil.

If you get right down to it, "overbore" is only relevant or descriptive with a certain length barrel, assuming that you have a slow enough powder available to take advantage of the case capacity while remaining within pressure limits in the first place.  If you can't fill the case with US869 or 20N29 without exceeding pressure limits, you have wasted case capacity and are truly overbore regardless of barrel length.  For example, if you have 22" of barrel to play with a 7mm RSAUM makes a lot of sense.  The cartridge was essentially designed around that barrel length.  Yes, you'd get more velocity with a longer barrel, but not a significant amount.  On the other hand, putting a 7mm Rem Mag in that same 22" barrel wastes a lot of potential and might leave you short of the SAUM in velocity from that barrel length, even though the RM has more case capacity.

 
True speed is a direct correlation with pressure on the back of the bullet.  But if you have a smaller starting surface area you have a smaller starting chamber volume, and therefor the pressure dissipation will be faster as the bullet moves down the barrel in regards to the percentage increase in total volume and a fast powder becomes necessary to make up for this dissipation at the start of the pressure curve, which makes the pressure curve drop more steeply and lower resultant velocity.
 
Get a three gallon air compressor and a five gallon air compressor and set them both at 90psi.  Hook up the same air ratchet to both.  That five gallon air compressor is going to give you more prolonged power solely based on the volume of air it can produce, even when both compressors kick on and make more pressure, that five gallon will already have the advantage. 
 
That initial volume of gasses in a larger cartridge allows for more maintained pressure, and in a smaller chamber, the resultant gases in a comparable expansion to the larger would cause a much larger and probably dangerous pressure spike as the bullet hits the lands.  Remember, the maximum pressure has nothing to do with velocity - the average pressure on the bullet is what causes velocity. 
 
And I completely agree that longer barrels do lessen muzzle blast and flash... I'm just saying most of us like a little percussion in the morning. 
 
 
I have to say I did go out and buy a little brother for my cz550 in .458 lott special(.450 marlin)  What a fun little gun.  1895GS, you want to make some quick boom thats the way to go. 

[gunnut69]

Bullet speed is relative to the pressure developed AND the length of time it is applied/maintained. The reason for slow burn rate powders.. and the large cases needed to hold them.. The bullet diameter has no relavance to the case volume. It does have relavency to bore volume and the relationship between case capacity and bore volume is what generates the term over bore... 
The analagy of differing size compressor tanks has no relavency unless we are considering pre-charged air rifles. Case capacities are only relavent if the pressure is already present when the round is fired. The burn rate characteristics of the powders in the cases indicated, as well as the weight of the bullets will define the pressure curve and its duration...thus the velocity.\
helotaxy seems to have given the best sense of knowledge of the term and interior balistcs in general. Kudos to him(her).

[Lloyd Smale]

Turning into a science lesson. thankfully i spent my younger days shooting and killing not studying math.

[Mikey]

I think a good example of overbore is joe biden - too big a mouth for his lips...............

[gunnut69]

Well hunting and killing doesn't require one to understand the how and why but it certainly improves ones abilities..
Mikey, great observation. I would add that in Washington, Bidens affliction is the norm..

[FPH]

Very informative.