I grabbed this from a thread posted by Geohans. Not sure how accurate it is.
EDIT, link to thread added:
http://www.ar15.com/forums/t_3_12/450544_Velocity_Barrel_Length.html
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
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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