Took the time out the other day to confirm that 62gr XM855 hits to the same point of aim as 69gr Sierra Match King (0.894" long).
This isn't a stretch because the steel portion of the SS109 design makes it light for its length (0.906").
The 77gr Sierra Match King that's in Mk 262 Mod 0 is 0.996" long. 1:9 doesn't typically stabilize it or the 1.15" long 63.7gr M856 Tracer.
Yet...
1:12 does stabilize the 0.91" long 54gr M196 Tracer and the 0.76" long 55gr M193 Ball. Hmmmmm.
Then why doesn't 1:12 stabilize M855 since it's the same length as M196?
In a word, velocity. M196 is exiting the muzzle at around 3,200 feet per second. M855 is just puttering around at 3,025. My favorite SBR load is at 2,850 fps. Mk 262 is even slower at 2,733 fps. Slower means you need more spin to get the same stabilization. The math is above my pay grade, I'm just regurgitating the conclusions.
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