Yes, I'm aware it looks vaguely like a P-90 with a hole in it.
This is my first lego gun that fires. It's a bolt-action, fires a 4-stud length Technic beam, the clip loads 1 round, the actual barrell and trigger are about the size of a pistol, and the firing pin sometimes flies out with the bullet, but that's ok because like I said, it's my first.
Max range is around 20 feet.
Firing mech. See the two dark grey bars in front of the firing pin? Those are attached to a rubber band (the same one that pulls the trigger forward.) When the trigger is depressed, they drop down, releasing the firing pin and shooting the bullet out of the short barrel.