Description after the jump:
I like Nerf guns and building Lego engineering projects, so when the idea came up to make a Lego robot fire a Nerf gun I couldn't resist. The Nerf Barricade was a clear choice among my Nerf blasters because it is semi-automatic, making the Lego component's job much easier. I rigged up a Lego mechanism that attached to the Barricade's handle and then I built the mechanism on to that.
The finger pulling the trigger is simulated by an axles with gears on it (to make it the right size and shape). That axle is attached to two long beams running around the handle of the gun through guide rails. Those beams are then attached to cams via shorter beams. To pull the trigger, I set up motors to turn the cams. If the two motors (one on each side) are in sync, the "finger" axle pulls back the trigger smoothly (as smoothly as a Barricade trigger can be pulled) and rapidly (depending on the speed of cam rotation).
I originally built this to work with my Power Functions XL motors, but they only pulled the trigger so the turret would buzz continually. That wasted battery power and made the turret bad for surprise attacks, so I switched the PF XL motors out for NXT motors and used the third NXT motor to flip the on/off switch. With a bit of clever programming the turret now revs up quickly before opening fire. In order to increase the rate of fire I geared the pulling motors for speed and to keep them in sync I linked them by a long axle.