On my SMG 60 I have a tapped back cap with a bolt in it. I then drilled a whole into the bolt so the spring will sit into the bolt. This way I can have a longer spring with drive in it.
Seems to work alright but just as Echo wrote, bad things will happen if your hammer hits the screw/bolt in the rva.
Bad things include cracking the cast receiver, possibly stripping the receiver threads, who knows what else.