I think Rhineland turned left when they should have turned right.
They should have made a mag adapter and conversion kit for shooting .30 carbine that you could just slap into an otherwise stock G3.
I'm not positive, but I think .30 carbine would stabilize fine in the G3's 1:12 barrel. The M1 carbine used a 1:20, but there are T/C contender barrels in .30 carbine that are only 1:10.
A semi-auto that could cycle .308, .22LR, and .30 carbine with just a swap of a few parts? Even Diane Feinstein would want one.