Actually, airsoft with barrel recoil is available at the moment. Look at the Taigen Tiger 1. They have built Tiger 1 tanks with airsoft barrel recoil and also sell Tiger 1 turrets with the same feature - I have two. Now, because of the advent of Clark boards and others that have a Barrel recoil feature built in, one only has to rig up a simple linkage from a servo to the gun barrel and perform some barrel mods/ machining, to create the same effect on other tanks. so far, I have 3 Tiger 1's, a Pershing M26, A Walker Bulldog M41 and my latest is a King Tiger, all with airsoft barrel recoil. I can tell you that it's pretty cool firing bb's with barrel recoil and I have no loss in shot velocity - in fact, the opposite!!! I intend to modify all of my BB tanks in this way.
Cheers
CaptB