I'd expect it to have a fluted chamber like so many other H&K's (and the clones). Pretty sure I would not want to reload/shoot that brass even after resizing.
20 lbs would put it at about the same weight as a "pig" aka M60. Not a weight that many M60 gunners enjoyed carrying, but some liked being M60 gunners and you couldn't tear them away from their gun if you tried.
I reload brass from my CETME, as long as it isn't too badly dented by the back edge of the port on ejection. The flutes are no problem.