Aren't the Marcolmar guns re-welded parts kits/scrap...? Would be nice to have something that isn't butchered up to meet the retarded US import regulations, but I guess beggars can't be choosers...
Marcolmar refinished the expensive bits (rollers bcgs and such) re-engineered springs and the magwell and completely redo the FCG and internals to prevent a lot of the milsurp spicy parts from dropping in.
Bars and islands inside the receiver and such
HMG guns are basically just parts built guns with little bars on the inside to prevent the spicy FCG from going in.