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Isn't part of the theory also that the twist rate has a lot to do with penetration???

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A long bullet, such as an overweight-for-caliber bullet, or a brass solid of standard weight, will tend to have more yaw (tipping) than a standard lead core bullet. This can cause the bullet to tumble early and not penetrate deeply. This effect has been seen with long monolithic bullets including brass solids of standard weight and Barnes X of standard weight. Using a faster than normal twist counteracts this tendency. The yaw or tipping of extra-long bullets which tends to cause them to tumble early is exaggerated when the bullet strikes the animal at an angle (or it it strikes the animal straight on but hits a bone within the animal that is angled).

If the bullet is overweight, then increased sectional density will tend to have somewhat of a counterbalancing effect. If the overweight bullet is a soft nose, then expansion of the bullet will quickly reduce its length and unless tumbling occurs very early then it may not happen at all. If the bullet is pointed, then the likelihood of tumbling is increased.

One way to counter the tendency of a long bullet to yaw excessively and tumble is to use a barrel with a faster than standard twist. Art Alphin of A-Square implemented faster than normal twists in all of his DG calibers in order to increase penetration depth.

Another way to deal with the tipping issue is to use a monolithic bullet that is about 10% lighter than standard for the caliber (such as 270 grains in the 375 H&H). That brings the bullet length back to normal for a standard twist barrel. Since the bullet is lighter, a higher than standard velocity can be achieved as well.

Another issue to keep in mind is that as velocity increases, excessive yaw becomes exaggerated. It's a bit like having an unbalanced tire. At 25 miles per hour it is not noticeable, but at 70 miles per hour it vibrates badly. Thus, appropriate twist rate for bullet length becomes even more important with higher velocity calibers.

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