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The 6.8 SPC would be a bad joke as a general-issue service round. Lower hit probability than the 5.56, with greater weight (30-40% more, actually weighs about the same as the 7.62x39) and more recoil, too.
Thanks to the AR's 2.26" COL it's limited to stubby bullets. Can't fit the ogive of a sleek, pointy bullet in such a short space. You can either match the speed of the 5.56 with a wiffle ball, or you can equal the 5.56 BCs but go several hundred feet per second slower. The 90 gr XM68GD actually has such a poor BC that 5.56 matches its energy by the time it's gone 200 yards.
View attachment 879644
It would be a step backwards towards the 7.92x33 and 7.62x39. All for a supposed (dubious) increase in "stopping power". I think the 6.8 can have a place at short-medium ranges, greater performance from SBRs for close protection, increased power and penetration for hunting, etc. But not as a widespread "5.56 replacement". Intentionally or not, this is reflected in the name—6.8 Special Purpose Cartridge.
110gr+ out of a 6.8 would be what one should judge it on. The 90gr would be CQB (used like the m193). The 6.8 shines at distance (with 110gr+) compared to any mag length 556 in regards to energy and it is close to the drop of the 77gr OTM and the same wind deflection.
Run some numbers on 115gr Federal bullets 0.401bc @2,550fps (16” barrel) against 77gr OTM 0.372 at 2,750fps (16” barrel)
There is a significant energy advantage over all ranges of the trajectory. Hit probability due to increased recoil would be the concern.
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