Had a chance to chronograph some ammo out of my 6.5" barreled AR.

Hope this helps


MK 262 VELOCITY DATA
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7.5" 2053 FPS ... 10.5" 2363 FPS
DIFFERENCE 310 FPS
DIFFERENCE/INCH 103 FPS

10.5" 2363 FPS ... 14.5" 2576 FPS
DIFFERENCE 213 FPS
DIFFERENCE/INCH 53 FPS

14.5" 2576 FPS ... 16" 2669 FPS
DIFFERENCE 93 FPS
DIFFERENCE/INCH 62 FPS

16" 2669 FPS ... 18" 2769 FPS
DIFFERENCE 100 FPS
DIFFERENCE/INCH 50 FPS

18" 2769 FPS ... 20" 2818 FPS
DIFFERENCE 49 FPS
DIFFERENCE/INCH 25 FPS



M855 VELOCITY DATA
__________________________

7.5" 2244 FPS ... 10.5" 2639 FPS
DIFFERENCE 395 FPS
DIFFERENCE/INCH 132 FPS

10.5" 2639 FPS ... 14.5" 2861 FPS
DIFFERENCE 222 FPS
DIFFERENCE/INCH 56 FPS

14.5" 2861 FPS ... 16" 2938 FPS
DIFFERENCE 77 FPS
DIFFERENCE/INCH 51 FPS

16" 2938 FPS ... 18" 3046 FPS
DIFFERENCE 108 FPS
DIFFERENCE/INCH 54 FPS

18" 3046 FPS ... 20" 3061 FPS
DIFFERENCE 15 FPS
DIFFERENCE/INCH 7.5 FPS


XM193 VELOCITY DATA
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7.5" 2364 FPS ... 10.5" 2755 FPS
DIFFERENCE 391 FPS
DIFFERENCE/INCH 130 FPS PER INCH

10.5" 2755 FPS ... 14.5" 2984 FPS
DIFFERENCE 229 FPS
DIFFERENCE/INCH 57 FPS PER INCH

14.5" 2984 FPS ... 16" 3075 FPS
DIFFERENCE 91 FPS
DIFFERENCE/INCH 61 FPS

16" 3075 FPS ... 18" 3245 FPS
DIFFERENCE 170 FPS
DIFFERENCE/INCH 85 FPS

18" 3245 ... 20" 3254 FPS
DIFFERENCE 9 FPS
DIFFERENCE/INCH 4.5 FPS

XM855

7.5" 2317 - AR Pistol
9" 2319 - HK53
11.5" 2716 - AR
16" 2945 - AR

Hmmmmm



You are reporting velocity diferences of ca 130 fps per inch when moving from 10.5" to 7.5" barrel length.  Yet my 6.5" velocity data is roughly 400 fps slower than your 7.5" velocities with the same bullet weight.



I suspect that the velocity change is not linear per inch as the barrel gets shorter.

Your velocities ate low even for a 6.5" barrel.

Great thread. Tag for more info.