Singling the doubelstrike and removing the insane amount of deadspace in the plunger tube and around the AR makes for a very nice, compact, single shot pistol. I've used the Magnum barrel shroud as a base and added the space for a petg barrel insert and a new plunger tube head.
1- You will need basic modding skills to do this
You literally saw off the front of the blaster, including the plunger tube, everything in front of the furthest point the plunger head reaches goes and the news front glued in place (the blaster remains dismantalable as the barrel becomes part of the plunger set up and not the shell).
2- Unless you really need to print at 100% for some reason I suggest minimal infil and extra wall thickness. The petg should be a tight ut managable fit, you need to file/sand it out a little if it varies with your printer.
3- Think before you cut, see pictures for rough guide. The painted image show's how things line up when I did things the hard way and made the shell separable, never again.