They could have redesigned the M1 carbine action to be select fire and vented the barrel foregrip assembly.
There is such a version. I had one in SVN. Had to tweak the disconnector to go full auto, not designed too well, The selector switch was right there on the left side of the receiver between wood and metal.
With that pissant round, it needed all the help it could get. Still a pipsqueak pistol round.