Bah, worked it out and realised I'd given no thought to the psi. I'm no expert so I'm expecting to get shot down but... I'm guessing that your gun rating is 34cfm at atmospheric pressure? Not 34cf of 120psi air, in which case when it vents to Earths atmosphere (about 14.7psi) it would of course accelerate.
Anyway
34 cubic feet = 962772.784 cubic centimeters.
6mm bore = 28.2743339mm2 in area so thats 2.82743339cm2
962772.784 / 2.82743339 = 340511.21678236953974714148792025
So that means we need to shift 340511.216(to 3d.p.) in a minute.
340511.216 = 2.1158386 miles per minute, * 60 =
126.950316mph...
Now correct me someone!