... and simply mount an automatic rifle on the top. That after all is where this ultimately is all leading but no one dare say it.
Well, at least in the US, it is incredibly expensive to attain any full auto weapon (can't be manufactured after 1986 for any civilian possession)....and with that scarcity factor, you might be a bit apprehensive to modify or alter a collectors piece.
Then, you have to go through the lengthy federal background checks and licensing to possess a full auto weapon.
And frankly, IMHO...the ATF may see what you are doing with it and reclassify it as AOW (Any Other Weapon)....which I think (not sure) requires more paperwork, and/or might be denied as something a regular US citizen civilian can own...?
That last part I don't know about....but have my suspicions.
People keep seeming to think it is easy to own a full auto weapon (Machine gun)...it really is not.
An AR-15 is not a "machine gun"/Assault Weapon/full auto weapon.
It is merely a semi-auto rifle....it goes bang once each time you pull the trigger.