FMJ yaw and upset performance

[JASmith]

No argument with that Stan, but it really doesn't have to - at closer ranges it merely has to show a substantial improvement over the close-range calibre (the 5.56mm) in barrier penetration and terminal effectiveness, which should be no problem!

I think Tony's point is on the mark. The challenge is to define the metrics so that we can go a tiny step beyond qualitative (read intuitive) comparisons and work toward quantitative assessments. We also need to remember that bullet design makes a huge difference. The good news here is that what is doable in 5.56 is also transferable to 6, 6.5, 7, and 7.62. The converse is largely true up to limits of scaling. For example, we won't necessarily be able to have the same ratio of meplat to bullet diameter in the 5.56 as we do for the 7.62 and maintain large-scale manufacturability.

This logic suggests that we ignore results that depend on a specific bullet design and focus on design potential as the caliber-based metric. For example, kinetic energy density is a good metric for comparing the potential for bullets of the same design to perforate thin barriers. Similarly, the momentum density is a good metric for penetration depth potential.

Lethality potential is a slightly more controversial subject. Fackler's and others studies show that the shape and size of the wound is an important indicator of average lethality. A lot of folks like to use kinetic energy to assess lethality. The problem is that KE does not give even a hint of the shape of the wound channel. For that reason, I lean toward a metric that captures both the penetration potential and the amount of material to cut a permanent wound channel. The metric is the product of the bullet mass and the momentum density. Go to http://shootersnotes.com/battle-rifle-ca...l-effects/ for more detail and a reference to Fackler's work.

My own assessments suggest that matching or exceeding the penetration and perforation potential of the 147 grain 7.62 bullet is easy in the smaller calibers. The lethality metric is more challenging because the smaller calibers carry a significant bullet mass penalty.

I've tried a couple of different metrics that use direct comparisons to existing rounds, e. g., M855 at 25 meters, M80 at 1100 meters, etc. , but at specified longer ranges. For example, one can ask the new cartridge to have a 600 meter lethality potential at leas as great as that of the M855 at 25 meters. These tend to be a challenge and the outcome suggests that the choice of threshold metric will have a tremendous influence on the calibers, bullet weights, and muzzle velocities that survive the filter.

I'm open to suggestions, and am intrigued by LR1955's suggestion to go 50% greater than the current standard (e. g., the M855 at the same range as the new cartridge). This would stress the perforation and penetration potential but might bring the required lethality metric more in balance with the other two.