You would definitly need a different trig. mech. 40 mm showers (if thats what you were thinking of using are under preasure and are hard to depress as is... buttt if you could find a strong enough trig mech. this would be a great idea.. especially if you had an insurgents game..
... could rig up a a little mech box or something and have the piston drives forward and hit the grenade