Why isn't comparison by bullet weight good enough?

[longwinters]

I keep reading posts comparing different caliber rifles.  For example: compare 270, 280 and 30-06.  When I look in my reloading books to compare velocity etc... between these fine calibers I make my comparisons using the same weight bullet for each.  Then look at velocities, bullet drop out to 300 yds etc...  However, it is sometimes pointed out that comparison using the same bullet weight is not correct.  You have to compare by B.C and S.D.  It makes no sense to me at all, that this should be the case.  I would think the important thing is mass and velocity.   So please educate me. :?  

Long

[victorcharlie]

I hear ya longwinters.....and agree 100%............I think most of these type questions are really just for conversation......There sure are a lot of these type question isn't there?  Personal preference and nostalgia come into play as well......availablity, popularity etc...etc....etc......

Now, how many spirits can dance on the head of a pin?  That's the important guestion! :-D  Answer this one without debate and you've done something......

[goodconcretecolor]

I have to, respecfully, disagree with VictorCharlie. When it comes to determining a trajectory, BC and velocity are the only pertinent factors. Of course SD is a factor in BC. This is not to say what you are doing is not a valid comparison, it is but it only goes so far. Sectional density is  important when you are looking at penetration. One or more legendary African hunters are know to have killed several elephant with a 7x57 mm Mauser which launched a 175 grain fmj bullet at 2300-2400 fps. This is similar velocity and sectional density to a .416 Rigby with a 400 grain bullet so the depth of penetration would be similar with the two guns. The .416 just makes a bigger hole.  Another example is 150 grain bullets in a 270 and a 30-06. The muzzle velocities are close but the 270 has a higher SD(and usually BC as well) and so would deliver higher velocity down range and better penetration at any range. 150 gr. 30-06 is considered a deer and antelope load while the 150 gr 270 is considered a good load for larger game such as elk.

[Iowegan]

Sectional density is computed from the bullet diameter vs the length. The longer the bullet (within the same caliber), the higher the SD. Velocity and BC is not a factor in the calculation. SD is the primary factor for penetration. Goodconcretecolor stated a perfect example for the 270 vs the 30-'06.

SD does make a big difference on terminal performance. On thin skinned animals such as a deer, you don't want high SD, else the bullet will go completely through the animal and not dissipate the full energy potential. With elk, moose, bear, and other large animals, the higher the SD, the better terminal performance you will get.

Ballistic coefficient is a thoretical comparison to a bullet with zero friction from the air. A bullet with a BC of .600 means it has the ability to overcome air friction .6 as well as a perfect bullet or you could also say it has .4 drag. BC not only determines bullet drop at different ranges, it also means it is less affected by cross winds. The higher the BC, the more velocity will be retained down range.

BCs posted in manufacturer's specs are always overstated because they don't take velocity and the distortion from the barrel's rifling into consideration. The only real way to compute BC is to measure the velocity at the muzzle and again at 100 yards, then plug these numbers into Ballistic Explorer software (or other computer programs). You will usually get an actual BC about 10% less than factory specs.

Ballistic Coefficient changes as the velocity changes. The slower a bullet travels, the lower the BC will be. You can test a bullet at the muzzle then 100, 200 and 300 yards and find the BC will continue to get lower as velocity drops off. If you shoot a .308 bullet in a 308 Win at 2700 fps, then shoot the same bullet in a 300 Win Mag at 3100 fps, both at 100 yards, the higher velocity from the 300 Win Mag will make the measured BC higher. All bullets have a "variable BC", some are are better some are worse.

Unfortunately, bullet manufacturers don't tell you this. They post the same fixed BC for a given bullet no matter what cartridge it is used in or the velocity. The only reference to velocity is for jacket sheading in the bore if driven too fast.

If you are shooting at distances of 100-200 yards, BC is really not that important. It really starts to make a big difference at distances beyond 300 yards.  Long range target shooters and varment hunters really take BC serious.


longwinters, When shooting deer in the jack pines of the UP, you probably won't get a shot far enough where SD or BC have much impact. The deer will not know the difference.

[victorcharlie]

Good concrete........respectfully back at ya....I admit I'm no rocket scientist.......heck, I'm not a nuclear physicist either......but do you really think the penetration is the same because the SD is the same?...........I think you have to figure "momentum" in to the equation..........A "woofle" ball (plastic ball and bat).....is the same size as a baseball....but not the same SD........SD = the ratio of a bullets weight in pounds to the square of its diameter in inches. Lets say you can throw the woofle ball 30 mph faster than a baseball.........Ask yourself which one will hurt the most, a baseball at 70 mph or a woofle ball at 100 mph...........Hit me with the woofle ball please...........Momentum, or the ability of an object in motion to resist change is a key factor in penetration.....Momentum= mass X velocity.........sure, if the mass and the velocity are equal between 2 bullets of different caliber, then the higher SD would have a small advantage........ but only if it can be used.....i.e....if both can penetrate 24 inches, but the deer is only 12 inches thick then it's a mute point.......

Now lets take the same woffle ball and baseball........throw them the same speed at a suspended 50lb steel plate.........which one will make the steel plate move the farthest?
 
All things considered......a .416 rigby will still be going long after the 7X57 has stopped...........

As for the 150 grain 30.06 being a deer bullet and a 150 .270 being an elk bullet........my train of thought is that bullet construction as well as bullet design would be the determining factor......the 150 grain 06 bullet my very well have a thinner jacket making it expand faster........while the .270 150 grain might be thicker jacketed and designed to expand slower......just a guess........my brain hurts now.......this is all to much for a poor ole dumb country boy from the back woods and hills of Tennessee............Do you supose Alvin York was thinking about this when he was charging that German machine gun nest?  Maybe I'll stay at the Holiday Inn express tonight~

How about one of you rocket scientist sanity checking me on this please?

Here's a good article on Terminal Ballistics.......there are so many varibles involved it's hard to predict what a particular bullet will do in a collision.....

http://www.rathcoombe.net/sci-tech/ballistics/methods.html

[longwinters]

Well, again I am no math wiz, but if a 30-06 (shooting a 150 gn bullet) will shoot somewhere around 280 yds with being 1.5" hign at 100 yds and a 270 will shoot about the same with the same weight bullet, you have all seen the trajectory charts,  then at least to me nothing else really matters.  Please forgive the thickness of my skull and my run on sentances.  It still boils down to mass and velocity in the hunting world.

Again I am open to being educated.

Long

[cwlongshot]

Good concrete........respectfully back at ya....I admit I'm no rocket scientist.......heck, I'm not a nuclear physicist either......but do you really think the penetration is the same because the SD is the same?...........I think you have to figure "momentum" in to the equation..........A "woofle" ball (plastic ball and bat).....is the same size as a baseball....but not the same SD........SD = the ratio of a bullets weight in pounds to the square of its diameter in inches. Lets say you can throw the woofle ball 30 mph faster than a baseball.........Ask yourself which one will hurt the most, a baseball at 70 mph or a woofle ball at 100 mph...........Hit me with the woofle ball please...........Momentum, or the ability of an object in motion to resist change is a key factor in penetration.....Momentum= mass X velocity.........sure, if the mass and the velocity are equal between 2 bullets of different caliber, then the higher SD would have a small advantage........ but only if it can be used.....i.e....if both can penetrate 24 inches, but the deer is only 12 inches thick then it's a mute point.......

Now lets take the same woffle ball and baseball........throw them the same speed at a suspended 50lb steel plate.........which one will make the steel plate move the farthest?
 
All things considered......a .416 rigby will still be going long after the 7X57 has stopped...........

As for the 150 grain 30.06 being a deer bullet and a 150 .270 being an elk bullet........my train of thought is that bullet construction as well as bullet design would be the determining factor......the 150 grain 06 bullet my very well have a thinner jacket making it expand faster........while the .270 150 grain might be thicker jacketed and designed to expand slower......just a guess........my brain hurts now.......this is all to much for a poor ole dumb country boy from the back woods and hills of Tennessee............Do you supose Alvin York was thinking about this when he was charging that German machine gun nest?  Maybe I'll stay at the Holiday Inn express tonight~

How about one of you rocket scientist sanity checking me on this please?

Here's a good article on Terminal Ballistics.......there are so many varibles involved it's hard to predict what a particular bullet will do in a collision.....

http://www.rathcoombe.net/sci-tech/ballistics/methods.html

 If I may...
 SD is a sacle used to determine bullets effects on game.
 BC is a scale to determine airs effects on a bullet.

Of course there are other varibles in how a bullet will perform on game. But using the same analogy, a 150 gr .308dia bullet, of any design is not a large big game bullet. Will it work, OF COURSE, is it designed for it NO. The 150gr .270 bullet of ANY design, is intended for game LARGER than the average Whitetail/Antilope. This is just bullet weight talking. the bullets SD will also back this up.
 Your wiffle to base ball comparison is apples and oranges in comparison.
 
 You noting momentum is a good thought and one many overlook. I also like the heavy bullet theory as upposed to the very high velocity.
 Momentum kinda rides the line between the two. By throwing one faster than the other you are representing that. But the differences between the two are more in line with a new 17M2 and the 45-70 shooting 500gr bullets!!!  I can only imagion how fast you would ned to push the 17 to even come close to the 45-70!!!!

[goodconcretecolor]

I stand by what I said on SD and penetration. It may seem to go against common sense but the laws of physics will win over common sense everytime. SD relates mass to cross sectional area and momentum per unit area is the prime factor in determining penetration.
By the way, I may not be a"rocket scientist" either, but, I am an engineer.

[goodconcretecolor]

Victorcharlie,
Excellent article. The author included the following equation derived from the data he generated:Penetration:     PEN (in) = Bullet Weight (lbs) x Impact Velocity (fps) / Meplat Diameter (in) / 5
This equation is a mathematical statement of what have been saying.

[Coyote Hunter]

longwinters –

There is nothing wrong with comparing different calibers by comparing their ability to handle bullets of similar weight – it depends on what you are looking for.  You might, for example want to compare the 75g V-Max bullets in the .243Win at 3400fps, the .257 Roberts at 3500fps and the .25-06 at 3700fps.  The .243 bullet has a S.D. of 0.181 and a B.C. of 0.330 while the .25 caliber bullet has a S.D. of 0.162 and a B.C. of 0.290.

Zeroing each for Maximum Point Blank Range for a 4” diameter target:

.243Win = Zero @ 238 yards, MPBR @ 277 yards, -3.6” @ 300, -13.7” @ 400, -30.5” @ 500
.257 Roberts = Zero @ 241 yards, MPBR @ 280 yards, -3.4” @ 300, -13.5” @ 400, -30.6” @ 500
.25-06 = Zero @ 254 yards, MPBR @ 295 yards, -2.4” @ 300, -11.1” @ 400, -26.0” @ 500

While the .243 bullet has the highest S.D. and B.C., the .257 Roberts with its extra 100fps shoots flatter out to 485 yards, at which point both bullets are down 27.5”.  But the .25-06 beats them both at all ranges.  All three, however, make fine varmint rigs with the 75g bullet.

Using Sectional Density when choosing a bullet for bigger game, although far from being a perfect tool, is useful.  Sectional Density simply describes the relationship between a bullets weight to its diameter.  The actual formula is: (weight in pounds)/(diameter squared).  

I just weighed the end of a flyswatter at 143.4g.  If it was round it would have an approximate diameter of 4.5”.  Its Sectional Density is therefore about 0.000990.  The Sectional Density of a 140g 7mm bullet is 0.248 or about 250.5 times more than that of the flyswatter.

Question:  For any given velocity, which would you rather be hit with – the flat side of the flyswatter or the pointy end of the 140g 7mm bullet?

Most people would choose the flyswatter.  Why?  Sectional Density.  (Although they may not have even heard the term “Sectional Density”, they intuitively understand the flyswatter is likely to do less harm, and why.)  Mass is important and so is velocity, but Sectional Density is critical in determining what happens on contact.

Obviously, Sectional Density is most useful when comparing bullets that are close to each other in diameter, velocity and construction.  For example, other factors being equal, a bullet with a larger Sectional Density can be expected to penetrate further than a bullet with a lower Sectional Density.  Consider the following:

.270” 150g = S.D. 0.279
7mm 150g = S.D. 0.266
.308” 150g = S.D. 0.226

7mm 160g = S.D. 0.283
.308” 170g = S.D. 0.271

Among the 150g bullets the .270" has the highest Sectional Density and can reasonably be expected to penetrate the furthest.  But the S.D. for the .270” 150g, 7mm 160g and .308” 180g are about the same and these bullets can be expected to penetrate about the same.

Again, Sectional Density is an imperfect tool.  For example, attempting to determine relative performance by comparing the Sectional Density of bullets with dramatically different characteristics is pointless.  A 95g .243” bullet has a S.D. of 0.230 while a 350g .458”has a S.D. of .238.  Similar S.D’s, but when it comes to dropping elk I’ll bet on the .458” 350g bullet every time.  

Going back to the bullets listed above, for any given velocity I would prefer the 180g .308” for elk because of its extra mass even though its Sectional Density is comparable to the 150g .270” and 160g 7mm bullets.  If I had to choose from the 150g bullets for elk, I would probably go with the .270.

[longwinters]

OK, thanks guys I feel educated.  Now when I think of all the other variables like bullet jacket thickness, internal design etc... I am glad someone else did all the math work and I can just read up on different bullets, load some up and go shooting/hunting.  

Long

[victorcharlie]

I guess the point I'd like to try to make is there are so many variables that are different with each shot on game that it is very difficult to predict what's going to happen when the bullet enters game.......... just the variation in tissue alone, then bones at different angles, bullet construction, how the bullet changes shape when contacting different internal organs.....there is just no way of knowing........any of the medium bores at close to the same velocity will give about the same results......
   
The difference between most medium bores at a stated velocity such as the 30/06, 280, and 270 is for the most part negligible..........shoot what  you shot best and makes you happy...........because the end result will most likely be the same.........
 
I'm a big hole guy myself.........for me, rather than going to say a 300 magnum for more power, I'd increase the bullet diameter and weight........more mass, not more velocity..........but that's me...........I'm sure there are a few who can use the extended range of the 300's but most of us probably can't........
 
Didn't Jack O'conner and Elmer Keith have a few spirited debates over the same subject many years ago?  Both point of views are valid and a case can be made for either.........