What's Wrong With .30 Caliber? By Bryan Litz

[jonoMT]

I'd like to try some heavier bullets but won't twist rate be a limiting factor? I currently have a .308 1:12.

Great article!

Jon

 

[KIWI AL]

great artical brian,down here in nz, the 7mm is the most popular for hunting considering most lr rifles are built light for packing in and the ballistic advantage that you coverd in your artical. most of us seem to shoot light projectiles compared to your artical as 162 gr amax is the most popular.

 

[Slim_T]

Brian, great article. I am a mechanical engineer and enjoy the technical discussion and your understanding of and experience with the subject. The scaled comparisons is exactly the type of information I need to assist in selecting a chambering begin my next rifle build.

I am just starting long-range target shooting at Quantico MCB in an effort to increase my long-range hunting ability. To that end, I have been practicing with a 300 WSM that I built as a compromise between elk hunting and target shooting. I shoot 190 SMK because of seating depth with my current throat and the fact that I want the rounds to feed from the blind magazine. I could increase the throat so that I can achieve max powder capacity with current 220, 240 gr VLDs, but I still want to chamber my favorite 180gr hunting bullets. I have made a study and also found that the heavier bullets are lacking in their form factor. I am glad to see that I arrived at a conclusion that is agreement with your more extensive experience and expertise.

After a couple and practices and shoots, I have been bitten by the target shooting bug and am now considering a purpose built rifle for target shooting competition. I was considering the 6.5 x 284 however the 7mm WSM is also interesting.

I make the following observations and comments with the understanding that Brian's article was directed at target shooting and not hunting. However another member made some comments/inquiries about downrange performance on game so I would like to weigh in on the hunting aspect of the .30 cal versus the smaller calibers while keeping in mind that the subject of bullet terminal performance on game is, due to lack of specific testing and data, a somewhat subjective and anecdotal driven subject.

While a smaller caliber bullet can certainly be selected for a high BC and launched at the required velocity to compete with or exceed .30 cals in the kinetic energy department downrange there are other factors to be considered. Terminal performance on game such as elk, moose, or bear is in addition to kinetic energy, heavily dependent on momentum which is a function of mass x velocity where mass has equal contribution to the momentum function value. If a lighter bullet strikes a bone with the same kinetic energy strikes as a heavier bullet but with less momentum, typically the lighter bullet will damage the bone, but deposit most of it's kinetic energy to the bone and surrounding tissue at that point since velocity is dramatically changed while the heavier bullet will likely damage the bone and continue on to the vital organs with more overall energy due to its' larger momentum value.

Thanks again Brian for the excellent article. I look forward to your next article.

P.S. I am glad to hear that girls make excellent shooters as I am trying start my daughter into the sport.

 

[BryanLitz]

Jon,
Our heavy .308's are being designed to work in a 1:10" twist.

SlimT,
Thank you for your clear explanation of KE and momentum. As you acknowledged, the stated focus of the article was target shooting where neither are important, but they are very important for this (hunting) crowd.
The differences between KE and momentum are not obvious. My usual momentum consideration is recoil which is proportional to momentum and not KE.
Your explanation of how the two are different on impact is something I've never considered and is quite enlightening.
Score 1 for the heavy calibers!

Thank you,
-Bryan

 

[Mysticplayer]

Bryan, that was a great article and one I will reference often to shooters on another site. There is a long standing belief that heavier bullets drift less in the wind. No mention of bullet form is ever discussed.

Your article nicely compares all cals and weights that produce similar ballistics. And from real world shooting, that comes very close to my reality too.

What I would like to see in the bullet industry is some standard for generating BC values so that shooters can get a fair comparison of potential ballistics.

Of course, I understand as a marketing tool, that number can make or break a shooters choice thus affecting sales. But varying G forms, ogive shapes, ways of measuring/generating BC's, the number is becoming more opinion then fact.

Could get some increased competition in making more aero bullets?

I like what you are doing at Bergers and noticed the BC values are changing more in line with what we see when a primer is popped. Great stuff.

I have also quickly moved away from the 30cal to the 22 and 6.5. New High BC bullets and a general hate for recoil has made this choice easy. I can expect that as new bullets in larger cals exploit the shapes used in smaller cals, the playing field will once again be leveled.

Larger heavier bullets will offer higher BCs so LR performance will be superior when launched at the same speed. Of course, recoil will also be higher but that is the price you pay.

Look forward to more articles and info from you...plus new offerings from BERGER.

Take care.

Jerry

PS can you explain why there seems to be an upper limit on how heavy/long we can make a bullet? 22cal seems to max at 90gr, 6mm at 115gr, 140ish for the 6.5's, etc

Is it a material strength issue? would going to a lighter core help?

would be wonderful to see a 150gr 6.5 with a BC of 0.65 but no one seems to want to make a VLD bullet heavier/longer.

 

[WildcatB]

What I would like to see in the bullet industry is some standard for generating BC values so that shooters can get a fair comparison of potential ballistics.

Mysticplayer,

This is a guess, and hopefully Bryan will correct me on this, but I think BC is a constant, and should be static from gun to gun, and even (when adjusted for) all other variables including pressure. However, in the real world, you'll get different measured values for different guns, different elevations (even when corrected for), plus other factors, for the same bullets. I think this is because, and again I'm guessing, the amount of time a bullet will take to stabilize, will change depending upon equipment and environmental factors. I've heard of a bullet that would stabilize at one elevation and key hole at another - same gun/load/bullet. I think all BC calculations assume that the bullet is stable right out of the barrel. But if it pitches and yaws for a while before is spirals tight, its velocity will decrease faster than it would if it was stable which should make a difference on measured BC.

Also, there is more than one way to skin a cat and I'm guessing that different bullet manufactures don't all use the same method for coming up with their BC's.

I enjoy Bryan's article's too. Hopefully he'll give us some more insight about differences in measured BC's.



Paul

 

[eddybo]

Thank You Bryan,

Great article, one of the best I have ever read here, and there have been some great articles on this site.

 

[BryanLitz]

What I would like to see in the bullet industry is some standard for generating BC values so that shooters can get a fair comparison of potential ballistics.

Jerry,
I'm finishing up a book that will include experimentally measured BC's for over 100 long range bullets that I've collected over the last 2 years, all measured using the same method. My plan is to have this book in print by this spring/summer.
As far as practical limits to bullet length, one challenge is the fabrication, specifically the jackets. Copper jackets are drawn from flat circles of copper into cups. The longer and deeper the cup is, the harder it is to make it a uniform thickness.
There are other challenges as well, such as max practical length of bearing surface you can scrape down a bore before copper fouling becomes prohibitive. The longer bullets need very fast twist rates, which exacerbate every component of dispersion that's related to spin rate (which is most of them). Then there's the flight quality of the bullet. Very long bullets are more 'tipsy'. Consider the extreme case of an arrow with no fletchings. You can spin that sucker as fast as the dickens, but it simply won't fly well because it's just too long. Bullets that are longer than the current 'heavy for caliber' bullets are not as bad as the arrow, but begin to approach that domain.

Paul,
Your belief that bullets have different BC's from all rifles is a common one. It's true that if a bullet is fired with marginal stability, it will have some pitching/yawing motion before it goes to sleep, and that will affect the BC. However, if the bullet is adequately stabilized which is the case for most accurate rifles having a twist equal to or faster than recommended, the bullets will emerge with adequate stability, and fly with a tight spiral and minimum drag.
In other words, it's possible to have a depressed BC from pitching/yawing, but only if there's a problem. If everything is right, a bullet will have the same BC from any rifle that imparts adequate stability.
Atmospheric variations will make a bullet fly differently of course. It's simply a matter of accounting for the variables that affect air density.
In cases of borderline stability, you can have a bullet that's well stabilized in thin air (high alt, warm air), and have it not be stable in denser (low alt cold) air. Again, if the bullet is properly stabilized, it can overcome the denser air, fly perfectly point forward, and only be affected by the very predictable effects of different air density.

As far as real world variations in BC, your more likely to see a problem between lots of a given bullet. For example, I've tested different lots of a given bullet and found up to ~4% difference in BC. The difference is usually always from wider or narrower meplat (tip) diameters which is the #1 dimension that vary's among lots and affects drag. The difference in meplat diameter should be obvious, in the case of 4% of BC variation, there was about 0.015" difference in tip diameter.

-Bryan

 

[WildcatB]

Paul,
Your belief that bullets have different BC's from all rifles is a common one.
-Bryan

My belief was that BC is a constant and does not vary between guns, but that the measured values can change depending upon how fast/slow a bullet goes to sleep. You basically confirmed what I was thinking - sort of. However, you're saying that while stabilization differences can cause measured differences in bc, that is not very common and that differences in measured BC's that we see are most likely due to bullet manufacturing variations between lots. Please correct me it I'm mistaken.



Paul

 

[yobuck]

very interesting article. it would seem weve come full circle.
the 6.5x300wby. is the cartridge that started it all, at least in pa. thanks to normas 6.5 139 gr. b.t. bullet.
fact is though it was an illegal bullet for hunting, at least in pa. due to its metal jacket. the 7mm 168 sierra smk, and the 162 hornady b.t. match derailed the 139 gr 6.5. using the 300wby. case.
i use the 200 gr smk in my 30x378, and find them much flatter than the 220, and 240 smks at 1500. i mean not even close.
yet they are about the same at 1700. all 3 would hit a large truck tire w/same amount of elevation added.
so i guess the velocity difference pays off.
however, the 338x416 and 300gr smk starting out 250 fps slower than my 30x378, will clean my clock at 1500.
so i would also appriciate a 30 cal vs 338 comparison.
i have been hesitant to go to heavier bullets due to my mountainside testing.


regarding the rising popularity of the small calibers at 1000 yd. matches, many shooters dont use them on windy days.

 

[Mysticplayer]

Byran, I figured manf had a large role in why we are seeing an upper limit in bullet weights.

Seems that once we need a 7 twist, things get real flacky. Which means, the 7mm and 30cal still have room to grow

I think a 190 to 200gr 7mm VLD with a BC around/over 0.74 would be interesting. Would be a very useable bullet for all sorts of applications and pushing them to 2900+fps would be a no brainer as far as cartridges/powders go.

Ever consider using a lighter core? or a different jacket material?

With a lighter core, you get the same BC but can now go faster.

Some polymers have pretty high melting temps so core failure would be greatly reduced.

it would require a major change in how we make bullets, but couldn't a material change push the limits further?

Jerry

 

[Buffalobob]

In order to increase the BC of a given bullet you must increase the mass without increasing the cross sectional area. Decreasing the mass will decrease the BC.

BSL135

Assuming constant form factors (drag profiles), heavy bullets will have higher BC's than lighter bullets of the same caliber.

RSI

The Coefficient of Drag (C.D.) for a bullet is an aerodynamic factor that relates air drag to air density, cross-sectional area, velocity and mass. One way to view C.D. is as the "generic indicator" of drag for any bullet of the same shape. Sectional Density (weight multiplied by it's frontal area) can then be used to relate the drag coefficient to different bullet sizes.

Sectional Density = (Wt. in Grains/7,000) / (Dia.* Dia.)

You can see from the formula that a 1 inch diameter, 1 pound bullet (7,000 gr.) would produce a sectional density of 1. Indeed the standard projectile for all drag functions always weighs 1 pound with a 1 inch diameter.

Another term occasionally found in load manuals is the bullet's "Form Factor". The form factor is simply the C.D. of a bullet divided by the C.D. of a pre-defined drag function's standard reference projectile.

Form Factor = (C.D. of any bullet) / (C.D. of the Defined 'G' Function Std. Bullet)

Ballistic Coefficients are then the ratio of velocity retardation due to air drag (or C.D.) for a particular bullet to that of its larger 'G' Model standard reference projectile. To relate the size of the bullet to that of the standard projectile we simply divide the bullet's sectional density by it's form factor.


Ballistic Coefficient = (Bullet Sectional Density) / (Bullet Form Factor)

Wiki

the formula for calculating the ballistic coefficient for a body is as follows:

BC = \frac{M}{C_d \times A} = \frac{\rho \times l}{C_d}

where:

* BC = ballistic coefficient
* M = mass
* A = cross-sectional area
* Cd = drag coefficient
* ρ (rho) = average density
* l = body length

 

[petenz]

As far as real world variations in BC, your more likely to see a problem between lots of a given bullet. For example, I've tested different lots of a given bullet and found up to ~4% difference in BC. The difference is usually always from wider or narrower meplat (tip) diameters which is the #1 dimension that vary's among lots and affects drag. The difference in meplat diameter should be obvious, in the case of 4% of BC variation, there was about 0.015" difference in tip diameter.

Polymer tips such as Amax/Vmax/Ballistic tips avoid this problem, correct?

 

[BryanLitz]

Polymer tips such as Amax/Vmax/Ballistic tips avoid this problem, correct?

Yes.

It's also commonly believed that polymer tips increase BC significantly because they're pointier and not blunt like a typical open tip bullet. They do have a little less drag, but it's not a big difference.
Ballistic Tips actually have higher drag than other bullets of similar weight and caliber because they lack an effective boat-tail. The boat-tail on many of the Vmax and Amax bullets are usually very steep as well, which hurts their effectiveness at reducing base drag. So ironically, many of the bullets with polymer tips which are thought to have higher BC's due to the tips, actually have lower BC's because of poor boat-tails.
The heavier Amax's (105 gr 6mm, 140 gr 6.5mm, 162 7mm, 208 .30 cal) are exceptions. They all have very good boat tails and are among the lowest drag bullets made.
The 117 grain 6mm DTAC bullets were also well designed, but I don't think they're being made anymore.

-Bryan