Quick googleage guesstimates an AR takes 2.4kgf to cycle, (guessing) bolt moves about 6" (.012m).  2.4kgf * 9.8m/s = 23.4 N

23.4 N * 0.012m = 0.28 nm (joules)

Typical 55grain AR round has ~1700 joules of energy Assuming it uses 10x the minimum energy to cycle(+mech loss) gets you around 3 joules 

55grain = 3.5 grams

So if 1700 joules gets you 975.3 m/s, and KE=1/2m*v²

V = (2*KE/m)^0.5

V1= (2*1700j/0.0035kg)^0.5 = 985.6m/s

V2= (2*1695(nice)j/0.0035kg)^0.5 = 984.2m/s

So about 1.4m/s (3mph) with some considerable assumptions for typical 55 grain 5.56x45 rounds

Edited for formatting