Author Topic: obturation (Read 6247 times)

coyotejoe · « **on:** February 07, 2011, 11:29:31 AM »

I've been reading that a bullet too hard for the velocity and pressure will lead the bore because it won't obturate. So why does a bullet need to obturate if it is .001" over groove diameter before being fired?

Veral · « **Reply #1 on:** February 07, 2011, 04:37:09 PM »

Ask that question to the guys who are saying obturation is mandatory and they will be stumped.

The only time obturation is necessary is if the bullets are undersize, and smart cast shooters don't use undersize bullets.

stubshaft · « **Reply #2 on:** February 07, 2011, 06:04:26 PM »

I think that they are confusing the fact that all bullets obturate to some degree, even if it is a miniscule amount. These same confused individuals have taken on a new theory of "launch" dynamics based on the pressure curve of the powder used?

coyotejoe · « **Reply #3 on:** February 08, 2011, 04:09:47 AM »

Well now I am confused. I bought the Beartooth Bullets Tech Guide and Marshal Stanton claims that to avoid leading the bullet hardness must closely match the pressure and velocity of the load. He says that a too hard bullet with a light load will lead the bore just like a too soft bullet. On another cast bullet website, "Missouri Bullets" IIRC, they even have a formula to calculate the desired brinell hardness for a given chamber pressure or desired pressure for a given hardness number.
These days I'm lazy and prefer to buy precast bullets but back 40 years ago I cast a lot and, living a few miles from a foundry, was able to buy Linotype metal. I cast most of my bullets from pure Linotype, what a beautiful bullet that made, and I don't recall having the bore leading problems I have now. Yet by the current thinking that bullet should have been too hard for even magnum revolver loads, which I never shoot anyway.
What the hay?

stubshaft · « **Reply #4 on:** February 09, 2011, 09:46:28 PM »

I have shot pure Lino bullets out of my 45 ACP at a whopping 850fps. Bullets were sized .4525 and according to what you read, they should have leaded the bore like crazy. There is no way that the pressure generated should match the BHN of the bullet. Problem is that my gun didn't read the article, it shoots into 1 1/2" 50 yds with a perfectly clean bore.

The gun will tell you what it likes.

Veral · « **Reply #5 on:** February 10, 2011, 05:37:18 PM »

The concept of balancing pressure to bullet hardness being a mandatory thing in preventing leading can be easily disproven by simply trying the hardest alloy you can come up with at low velocities. I've shot 30 +BHN heat treated bullets at 700 and 800 fps from 38 brass in a 357 with absolutely no leading. But the bullets have to fill the gun before the primer pops. Maybe high quality lube is very important.

I hope you enjoyed Marshalls plagerized version of my book. I guess he feels important selling it though.

coyotejoe · « **Reply #6 on:** February 11, 2011, 03:50:31 AM »

Thanks Vernal, I just can't buy into that "too hard" business, it makes no sense to me and it's sonething I just never heard of before a few years ago. Sometimes it seems the less sense a thing makes the more people want to believe it.

pourboy · « **Reply #7 on:** February 11, 2011, 04:23:04 PM »

In order for obturation to occur, first there must be sufficient space for it to take place. I don't see how that can happen if the bullet is already at, or slightly larger than the bore diameter. I once cast a bunch of 45 ACP & 40 S&W bullets from what must have been pure monotype or something even harder. They were a pain to size, but didn't lead the bore at all. I normally only size bullets just slightly, enough to lube them and make sure the sides are basically parallel with no major high spots. Anyway, I never did have any leading with them at all, even with powder puff loads. I did have one issue however, the bullets were so hard that they fractured at the taper crimp, and as they fed from the magazine, the nose of the bullet would fly out the ejection port as the cartridge chambered. It was kind of neat in a way. The remaining flush seated full wadcutter would chamber without difficulty and was really accurate. I used the rest of that batch of alloy to sweeten soft mystery alloys until I finally used it all up.

pat i. · « **Reply #8 on:** February 16, 2011, 01:25:50 PM »

Quote from: coyotejoe on February 11, 2011, 03:50:31 AM

Sometimes it seems the less sense a thing makes the more people want to believe it.

Truer words were never spoken. Only thing better is seeing someone complicate something as simple as pouring melted lead into a mould past the point of human understanding and watching people fall all over themselves telling him what a genius he is. Of course they don't know what the heck he's talking about any more than the genius but why let a little thing like that bother anyone.

Sorry for sticking my two cents in here Veral and I know it's not the intent of the forum but key board experts and their half baked theories are a pet peeve of mine.

azvaquero · « **Reply #9 on:** March 18, 2011, 08:04:49 AM »

OK, now I have a question, and this is tied in with the discussion under "Fire-lapped (45 colt) and still have leading issues".

If a bullet must seal gas before it even begins moving (while still inside the brass), and since it must be somewhat undersized in order to chamber (.002" chamber clearance), obturation IS necessary, even if only in the chamber. Correct? Even a minor .002" leak is a huge leak with 14,000 psi or greater behind it, and if lube is being blown out of the grooves, so much the worse. How could a bullet EVER be large enough to provide a complete seal from the moment of powder ignition? You would not be able to chamber it without a hammer. OBTURATION IS ALWAYS NECESSARY. Correct? This is not a minor problem either -- this is where bullet erosion and leading begins, and I believe where the majority of throat and forcing cone leading occurs. Veral, please chime in and let us know your thoughts. Thanks.

I'm harping on this problem because I have had leading issues in .45 Colt for a long, long time, and have tried just about everything possible, including the good "blue" lube Veral sells.

coyotejoe · « **Reply #10 on:** March 18, 2011, 09:48:34 AM »

You're not alone, I've experienced at least some leading in every revolver in which I've ever fired lead bullets. In fact for .357 mag I have just written off lead bullets and shoot jacketed exclusively, even for .38 special loads. I've never had a problem with rifles, although I've never loaded cast rifle bullets very hot and most that I have shot in rifles were gas checked. I've never tried cast bullets in auto pistols but revolvers with lead seem to be a problem for me. I have learned quite a bit about bullet fit of chamber throats and bores and fire-lapping, all of which have lessened leading but I still see leading at the first inch of rifling after as few as a dozen rounds and the Lewis lead remover always brings out lots of shiny flakes. But I'm not sure "obturation" has anything to do with it, if anything maybe "over-obturation" at the forcing cone.

pourboy · « **Reply #11 on:** March 19, 2011, 06:23:25 AM »

I do experiment a lot, but keep coming back to one simple guideline. I have had my greatest successes using the largest cast bullets that would safely & reliably chamber in my guns. Most chambers seem to be oversized to start with. I don't want to see them jammed into the chamber tightly, but they shouldn't rattle around loose in the chamber. This meshes neatly with sizing bullets to fit the throats. Interestingly, most semi-autos with production barrels seem to have sloppy chambers & wierd throating. And what class of firearm is the most challenging to cast for?

azvaquero · « **Reply #12 on:** March 24, 2011, 07:37:24 AM »

OK, pourboy, agreed. The larger the slug (assuming it will chamber), the better. But wouldn't obturation still be necessary? (as much as .002" on the base?) After all, if it falls into the chamber, it is obviously too small for a seal. It seems obturation would always be necessary, because .002" allows a lot of leakage with 14,000 psi or greater. Lube would be blown away, and gasses would erode the base and sides. In fact, some of this leakage is going to occur anyway before pressure builds up enough to obturate the base. Veral, do you agree with this assessment? SOME bullet "bump up" is always necessary, probably at least .002" under ideal conditions.

Veral · « **Reply #13 on:** March 24, 2011, 06:09:56 PM »

Size of the gap is crucial so far as how much gas can flow past, with burning powder helping to cork the space a little. One law of physics is that gas flow throgh a gap increases four times when the space width increases once. So a close fit to start retards gas flow enough to allow the bullet base to obturate enough to seal off blowby before enough gas passes to wash lube out.

So far as leading problems with revolvers, there should be non if the bore is lapped, cylinder throats at least as large as groove, and preferably slightly over, and if bullets fit close enough to create a tight seal. When fitted to a slip fit to cylinder throats, gas blowby is checked completely, if the first drive band is inside the throats when the rounds are chambered. If the drive band is back from the throat, all the sealing has to take place inside the chamber area until the bullet moves far enough to get it's cylinder throat seal.

With the right fit to prevent gas blowby, a smooth bore, etc, and LBT bullet lube, one should be able to run any of the magnum revovlers with near magnum loads using plainbase bullets. That will mean velocities well over what jacketed bullets produce at similar pressures, so I'm not talking anemic loads. With gas checked bullets, we can stress the guns pressure limits severely before leading starts.

With any other brand of lube, expect leading, especially in the forcing cone. That has been my experiance.

azvaquero · « **Reply #14 on:** March 25, 2011, 11:44:54 AM »

Thanks Veral.

So what this is telling us is that for most shooters with non-custom "oversize" chambers (most stock Rugers running on the order of .483 at the front portion of the chamber), and assuming a .451" bore with .452" throats, we would want a bullet sized to .459" on all driving bands behind the crimp, and (based on your 3/24/11 comment Veral, which I had often wondered about myself), a nice wide front driving band (say .125" wide or wider?) cast at .451", for a slip fit into the throats.

The driving bands below the crimp, being sized .459", would give a finished cartridge OD of .481" (.459" + .022", Starline brass being .011" per side (you would vary this if your brass is thicker or thinner)). This would slip fit into a .483" chamber with .002" clearance and provide a good seal with minimal obturation necessary. Further, the .451" front driving band sitting in the throat would hold forward gas leakage to a minimum, which would not only protect the lube and sides of the bullet in the few moments before rear driving band obturation, but would also help align the bullet and guide it straight as it moves forward and sizes down the rear two driving bands by .007".

Is this the ideal setup Veral? Have you ever experimented with a bullet designed this way?

Veral · « **Reply #15 on:** March 29, 2011, 08:55:00 PM »

If the bullet is heavy, and has a drive band extending out into the cylinder throats, it MUST be size to be able to control the fit precisely. This type bullet isn't what gives the problems with leading, but the short ones which don't have a seal in the cylinder throats when they are chambered. For short bullets, the drive band forward of the crimp should be short enough that it doesn't contact the cylinder throats, and the entire body made large enough to fill the chamber so it stays straight on takeoff and provides a seal from the git go.

Your statement, 'a few moments before obturation takes place' is a slight miscomprehension of time span, me thinks. Milliseconds is the right word, and not many of them!

coyotejoe · « **Reply #16 on:** April 03, 2011, 07:15:25 AM »

Well how about special brass in magnum chambers? I mostly shoot upper level .44 special loads in my .44 mag and a lot of .38 specials in my .357. Would it reduce leading if I put my special loads in magnum length brass?

Veral · « **Reply #17 on:** April 05, 2011, 08:05:12 PM »

I've shot a lot of 38 loads of all levels of power through 357 chambers with no leading problems, and quite a few 44 special loads in 44 magnums with no problems. This both in revolvers and fixed barrels.

If you have no problems using special brass, go for it, but if you run into leading, the first thing to try might be switching to magnum brass.

wncchester · « **Reply #18 on:** April 18, 2011, 08:56:04 AM »

Obturation simply means the bullet base gets pushed forward enough to fill the bore. Any bullet even a smidgeon over bore size starts that way.

The ONLY time I've found cast bullets not to obturate is when firing very hard but undersized bullets with light to moderate charges; then they will gas cut like crazy and lead plate a bore in a hurry no matter how good the lube is! With full power loads, rifle and handgun, even most undersized jacketed bullets will easily swell enough to obturate.

Veral · « **Reply #19 on:** April 18, 2011, 06:44:08 PM »

Good explaination, which might be more clear than the way I would write it.

azvaquero · « **Reply #20 on:** April 24, 2011, 07:22:09 PM »

Yes Winchester, but here we are talking about obturation BEFORE the bore -- in fact, before the bullet even enters the throats, before they even leave the chamber in fact. Leading sprayed into the forcing cone and/or throats is not necessarily a bore issue at all, but a symptom of gas cutting before the bullet enters the bore. Also, bullets in the chamber aren't and can't be a "smidgeon" oversize (of chamber diameter) or they wouldn't enter the chamber in the first place, unless you like to chamber rounds with a ball peen hammer. I don't get that much leading at all past the first inch or so in the bore. It is all up front in the throats, forcing cone, and rifling under the threads (yes, I am aware of the compression issue which firelapping removes, and have firelapped several guns with improvement in the bore, but sadly no reduction in up front leading, because again, it's not a bore issue). What the .45 Colt shooters are experiencing and what they are complaining about is leading NOT with superhard slugs (which would of course cause a problem), but leading with fairly soft slugs. The leading I have experienced is from air cooled WW over charges of, say, 7.5, 8.5, 9.0, and even 9.5 or 10.0 gr. Unique. In other words, the whole gamut. Mild to hot. The difficulty, as Veral is explaining it, is getting a .452" slug to obturate close to .009 of an inch before it moves out of the mouth of the brass! Not an easy feat, which is why he is telling us that a .459" bullet is the solution. Thanks for the input though, much appreciated.

painted horse · « **Reply #21 on:** May 10, 2011, 08:04:02 PM »

Uhhh.. you're saying you want to size your 45 colt bullets to .459 and shoot them in a .451 bore??Yowza, that just don't seem like a good idea to me.

Veral · « **Reply #22 on:** May 13, 2011, 08:14:28 PM »

You could probably detect a difference in pressure if shooting jacketed, if the cylinder throats were a lot smaller than bullets, but you wouldn't be able to with cast, even the hardest cast you can make.

BAGTIC · « **Reply #23 on:** August 06, 2011, 10:08:34 AM »

Bullets can not be bigger than the chamber or they would not fit in the chamber. They can be bigger than the chamber throat. The chamber throat will size them down some and when they finally enter the bore it will size them further.
Look at it like drawing a wire down to a smaller size. A large wire is pulled though a series of successively smaller apertures.
There have been many published experiments where the barrel is removed from a revolver and the bullet launched directly from the cylinder Lead bullet consistently expand without encountering any resistance from a barrel. Not only does the base expand but also the body while the ogive is typically foreshortened by the setback.

.22-5-40 · « **Reply #24 on:** August 11, 2011, 07:14:15 PM »

Hello, everyone..Very interesting posts! I see alot of folks talking about bullets over groove dia..I am talking about rifles & why the need for obturation? But how about the throat dia.? this is usually over groove dia. and this must be sealed to prevent gas cutting & blow by. I ran into a problem with hard bullets in a .22 Hornet. 1-14" Shillen 26" match grade chrome moly, I cast up various bullets using Steriotype alloy..this stuff is harder than lino..15% sb. It shot the little Lyman 225415 old style (45gr.) great, but anything heaver shot patterns, even with increased powder charges. Now the std. accuracy load with this rifle is a 55gr. Leeth nose-pour. No leading with the heaver slugs but no accuracy..Wonder why?

coyotejoe · « **Reply #25 on:** August 12, 2011, 06:29:25 AM »

I would guess the 14" twist is not adequate for a 55 grain bullet, I've never gotten accuracy from jacketed bullets heavier than 50 grains from the couple of Hornets I've owned and 45 grain bullets were always better.

Veral · « **Reply #26 on:** August 24, 2011, 08:57:39 PM »

Re: Bagtip post. The forward part of cast bullets does not expand unless the alloy is softer than I would recommend. In other words, if the nose or ogive expands the bullets are too soft to perform well at the pressures of interest. I'm talking revolver or rifle.

I've made quite a few molds for hornets, and have learned from feedback and by measuring throat slugs, that the reason Hornest are problematic with cast is normally because the chamber neck area and throats are very large. With the slow twist used in most, bullet weights of 40 and 45 gr are about max that will stabilize, and the short bullets will not get started straight into the rifling,

The ideal answer is tighter chamber and throat dimensions if building a gun, or shoot oversize. Best to send a throat slug and let me make a mold to fit if a worn gun is being fitted. Which is the case with probably 90% of the Hornets I've made molds for.

.22-5-40 · « **Reply #27 on:** August 25, 2011, 03:34:04 PM »

Hello, coyotejoe, No thats not it..those 55gr. leeth bullets are grouping well under 1/2" AT 100yds. But not with the hard stuff..but the little 45 did great with it? Something I'll have to play with somemore.

BAGTIC · « **Reply #28 on:** October 01, 2011, 07:54:32 PM »

I don't size any of my lead buillets. As long as they chamber I shoot them as is. The best sizing die ever made is the firearm itself. It always gets the diamter exactly right for its needs.
If the bullet doesn't completely fill the bore right down to the botton of the grooves gas will leak. Not good.
For lead to come off the bullet and attach to the barrel there must be a space. Everyone knows how much easier it is to strip a bolt/nut thread if the bolt is slighly undersized. An undersized bullet is no different. If a bullet is undersized it is more prone to strip and deposit lead in the nut (barrel grooves).
Also, if the bullet is not tight in the bore the bullet can cock. To be accurate a bullet should be oriented to the centerline of the bore. If it is not there goes accuracy
Forget the talk about leading being caused by the lead bullet melting or vaporizing. If it were being spread in a gaseous or liquid form it would be uniformly distributed in the barrel on top of the lands as well as in the grooves. I have yet in 50 years seen uniformly applied leading. Except once when I shot a bunch of Speer swaged .357 through my Trooper. In less that 12 shot the last two inches of the six inch bore was so heavily 'plastered' with lead that the riflings were totally invisible.
.

Frank2 · « **Reply #29 on:** November 21, 2011, 05:35:20 PM »

Bagtic:

Quote

I don't size any of my lead buillets. As long as they chamber I shoot them as is. The best sizing die ever made is the firearm itself. It always gets the diamter exactly right for its needs.

Veral:

Quote

If the bullet is heavy, and has a drive band extending out into the cylinder throats, it MUST be size to be able to control the fit precisely. This type bullet isn't what gives the problems with leading, but the short ones which don't have a seal in the cylinder throats when they are chambered. For short bullets, the drive band forward of the crimp should be short enough that it doesn't contact the cylinder throats, and the entire body made large enough to fill the chamber so it stays straight on takeoff and provides a seal from the git go.

Bingo!