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

 
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  #22  
Old 02-09-2009, 02:58 PM
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Re: What's Wrong With .30 Caliber? By Bryan Litz

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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
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  #23  
Old 02-09-2009, 04:38 PM
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Re: What's Wrong With .30 Caliber? By Bryan Litz

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Originally Posted by bsl135 View Post

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

Last edited by WildcatB; 02-10-2009 at 12:58 AM. Reason: typo
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  #24  
Old 02-10-2009, 12:05 AM
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Re: What's Wrong With .30 Caliber? By Bryan Litz

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.
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Old 02-10-2009, 11:38 AM
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Re: What's Wrong With .30 Caliber? By Bryan Litz

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
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  #26  
Old 02-14-2009, 05:06 PM
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Re: What's Wrong With .30 Caliber? By Bryan Litz

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

Quote:
Assuming constant form factors (drag profiles), heavy bullets will have higher BCs than lighter bullets of the same caliber.
RSI
Quote:
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

Quote:
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
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  #27  
Old 02-15-2009, 05:29 AM
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Re: What's Wrong With .30 Caliber? By Bryan Litz

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Originally Posted by bsl135 View Post

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?
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Old 02-15-2009, 09:43 AM
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Re: What's Wrong With .30 Caliber? By Bryan Litz

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Originally Posted by petenz View Post
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
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