Pretty close to an analysis I did for time difference between crack and pop for the first shot. I got approximately 2750-2870fps.
It will also depend on the type of 5.56 ammo being used. See here for some examples: 5.56x45 vs 7.62x39 - Cartridge Comparison - Sniper Country The velocities there are probably all for 20-inch barrels, so have to be adjusted accordingly, but 2800fps is definitely a good benchmark.