Can anyone tell me the typical muzzle velocity for a factory 7.62x39 122gr FMJ round out of a short (say 8”) barrel? Thanks

Tag.  I’ve wondered the same.  

I had too many variables with barrel (308 vs 311) and way too much ammo variability.    

Ultimately I decided the only real answer would be to do the testing myself.   I need to buy a Garmin device.  

I can test with my barrel, specific ammo and my suppressor.

From a youtoob video I found from a couple years ago, testing with multiple rifles with different barrel lengths, thankfully somebody summarized results in the comments:

IWI Galil Ace Gen 2 8.3'', 1992 fps
Radical Firearms HBAR AR pistol 10.5'', 2189 fps
Pioneer Arms Hellpup 11.73'', 2180 fps
M+M M10-X 12.5'', 2200 fps
Meridian Defense Volk 14.5'', 2263 fps
Zastava Zpap M70 16.3'', 2344 fps
CMMG Mk47 Resolute 16.1'', 2365 fps
Chinese SKS (Type 56) 20'', 2367 fps
Century Arms AES10B 21.5'' civilian RPK clone, 2450 fps

Looks like velocity fell off a cliff going from 10.5 down to 8.3, but could be something about that particular firearm. A better test would be trying different length barrels all made the same by the same manufacturer and all used on the same rifle, an AR would be perfect for that.

But still, even if all you get is 2000 fps that's a pretty good thump at short range.

ETA: Also interesting, from some other charts I found that measured from 16" barrels up to 24 inch barrels, velocity peaks around 23 inches, in both cases velocity fell off some at 24 inches. So looks like 23 inches or so and the round peaks ballistically and velocity will fall after that.