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Discussion Starter · #1 ·
If there is a "fountain of youth" in ballistics, it is the ability to make little bullets behave like big bullets. It is not a new concept, rather an ancient ballisticians' dream. From the .75 caliber "Brown Bess" and the .69 caliber Charleville, popular calibers have diminished to .223 for the military and .17 for civilians.

When you reduce a bullet to 1/3 of its original weight, you are left with 1/3 of the original kinetic energy. To maintain the same kinetic energy, the new bullet would need about 73 percent more speed. But, what if you could make the little bullet twice as efficient as the big bullet? Then it might only need 25 to 30 percent more speed to equal the big bullet's performance. If this sounds like the .45-70 v. .30-40, .308 v. .223 or the .223 v. .17, then you're right. That's how those changes came to pass.

Prior to WWII, the U.S. Army convened pig and goat boards to evaluate a new caliber for the soon to be adopted semiauto rifle. As we all know the M1 Garand was produced in .30-06, but .25 and .276 rifles were also tested. The tests concluded that within 500 yards the .25 caliber bullet starting at 2550 fps was more effective than the .30-06 bullet starting at 2900 fps. How could this be?

The tests were not flawed, rather the .25 bullet was designed to take advantage the yaw that occurs when a bullet enters tissue at a high velocity. Of course .30 caliber bullets can be designed the same way. The smaller caliber offers reduced fatigue from firing, plus lighter weight.

When a stable .30 bullet strikes an enemy soldier, it may only use 10 or 15 percent of its kinetic energy creating the wound. The secret to making little bullets behave like big bullets is to create a high drag on impact. Yawing both increases the area pressing against the tissue and the bluntness of the bullet. Expanding, yawing and other high drag bullets may be able to use 50-70 percent of their kinetic energy for wounding. Expanding and fragmenting bullets have been unacceptable to many nations since The Hague Convention of 1902. This leaves yawing as a wound mechanism in military bullets.

However, we've all know about yawing bullets for some time. So, how do they work? They shift the bullet's center of gravity further rearward than it might ordinarily be. The British used an aluminum nose in their Mk VII .303's, the Russian 5.45x39.5mm and Chinese 5.8mm use air spaces, but there is a simpler way...make the bullet longer. Accuracy is maintained with a tighter twist. An enemy soldier will be 900-1000 times denser than air, so the twist will not stop the bullet from yawing. The twist for a 55 grain .223 would have to be changed from 1 turn in 12 inches to about 1 turn in 3/8 inch to remain stable in tissue! At high velocities, a sharply yawing bullet may fragment, expending 100 percent of its energy wounding. That's how the little bullets do it.
 

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Just out of curiosity, what does the term "over stabilized" mean in relation to a projectile? I assume it has something to do with too fast of a twist rate? The implication was that it was a bad thing to have......?
 

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Discussion Starter · #4 ·
Rich, there is a math formula that is used to determine the twist needed to make a bullet stable. This formula is known as the stability factor (SF). If the SF is too low, the bullet will not fly straight.

Remember though, the bullet must also remain point-forward as its trajectory curves. The bullet leaves the muzzle pointed in the same direction as the muzzle. If the twist is too fast the bullet will not gently turn through its trajectory, always pointing in the direction of travel.

When this happens in long rang shooting the bullet tries to arrive with an upward yaw, "nose up". Thus, at long ranges the overstabilized bullet will have more drag and will drift to one side due to the gyroscopic interaction of the twist with the nose up bullet.

Summation: at long ranges, overstabilized causes...

1) too much drag

2) excessive sideways drift (gyroscopic)

3) more nutation (spiraling)

4) increased wind deflection
 

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Discussion Starter · #5 ·
TODD 3465 said:
In this vein do you have any current data/reports on the 77grOTM bullets being used in Afghanistan?
Yes, I know a bit about this round.

It was not originally purchased for Afganistan, but for use in another region pre-9/11. Fired from M4's, it is able to penetrate the exterior walls of ordinary houses and some light commercial buildings and still be effective. Reports from Afganistan state that it is an improvement over most .223 rounds.
 

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Discussion Starter · #6 ·
TODD, let me add that I have never seen a 77 gn OTM (OTM = open tip, match) mushroom in a testing medium. At close ranges they yaw and, more often than not, break into two large chunks, plus some insignificantly smaller pieces.
 

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Sounds good as I'm thinking of loading a bunch up to put back. I already did some load testing and found they grouped well out of my AR's.
 
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