Tony Williams Wrote:Since the sole purpose of replacing the 7.62mm is to reduce its weight (its performance is fine), and the more the performance exceeds that of the M80 the heavier the cartridge will be, my view is that the new round should match, as closely as possible, the trajectory and wind-drift of the M80 at long range. The round should be the lightest which can achieve that.
Tony Williams Wrote:My thinking on this subject has been gradually evolving. I'm now inclined to think that while 6.8x46 may be about right, the case diameter should be wider than the 6.8x43, for two reasons: the need to accommodate significantly longer copper+steel bullets, especially tracers, which will occupy more of the case capacity; and the desirability of allowing for the future use of part-polymer cases, which may have thicker walls than metal ones, thereby further reducing propellant capacity.The second post above pretty much mandates use of the 0.445" diameter case of the 7x46 UIAC.
To meet the requirements in the first post calls for bullets with approximately the same sectional density and ballistic coefficient as for 147gr 7.62 Ball. That's roughly 130gr/7mm, 120gr/6.8mm and 110gr/6.5mm, all of which would duplicate 7.62 muzzle (and downrange) velocity, trajectory and wind drift, when loaded in the UIAC case.
If weight reduction is the sole priority, clearly 6.5mm is the best choice. 6.5x46 is 30% lighter than 7.62x51, whereas 7x46 weighs 25% less than 7.62x51.
What concerns me is that 6.5mm almost certainly would be inferior to 6.8mm and 7mm in regard to target effects. But, whether or not the difference would be enough to matter, is something that could only be determined by testing.