The shotgun packs a hell of a wallop. The pump is my back up gun because of it. The semis are a LOT gentler to shoot. Even still, it's just a different weapon system that requires training/"getting used to".
Machine guns are another ball of wax as well; different "manual of arms" again, and different techniques, etc. are required. The typical "bladed" stance from the "old school" of rifle shooting is "unhelpful" when shooting MG's. If you thought it was tricky to control now, you should try it in stock configuration sometime!
I'm redoing the stock mount on it now, however, and that should help to reduce what bit of muzzle-rise is left in it. Currently the stock sits about 1" below the bore center-line, which gives it some leverage to climb up as you shoot. The weight of it helps a *LOT* to keep it down though.
The stock configuration drops maybe 3 or 4lbs of weight off the gun, and moves the stock another inch or so lower. Plus, there's really nothing to hold on to in the front (unless the factory suppressor is mounted). Between that and running about 50% faster, it makes for a scary combination with new shooters behind the trigger!