How's that M4/ M16 thingy working out?
As for the M9, the gun was large and heavy. I personally couldn't hit a damn thing with one, but I know people who like them. I didn't.
The plastic parts on the m16/m4 are stock and pistol grip. The hand guards have mostly been changed to alum.quad rails. The old ones had been known to crack. The stock is solid and has the buffer tube inside. I'm not a fan of a plastic frame on a military sidearm. The M9 was a fine back up weapon.