Oh dear really tinbum Sir, I say lay aside the rat-a-tat-tat. Purchase an electrically-operated airsoft 'maschinengewehr', an M60 or something and get it out of your system.

What you actually want to do is make a rolling-block PCP in the manner of the Shiloh or the Farquharson rifles. You could conceal the air cylinder in the butt a la the Phoenix and use a nice thick bull barrel up front to balance it out. Only make them in .25 calibre and FAC power levels of course.

Now that would be a PCP special.