It is worth pointing out that many long guns have plastic stocks these days, and some, such as the M14 and the SLR in British service, eventually had their wooden stocks replaced with plastic. But that was replacing wood, not metal, of course. Not the same thing but still, it must say something about the suitability of whatever kind of plastic it is.
It is also worth pointing out another law in play here. Parkinson's Law, about the behavior of people on committees, is clearly being demonstrated when the amount of discussion about a pistol is greater than the discussion about a rifle (barely) and even more so than the discussion about the machine gun. The amount of discussion is inversely related to both the monetary value of the weapon and the combat value of the weapon as well. It's still an interesting discussion, though. Typically, handguns are the last things to be replaced and often have the longest service life of any weapon, although the M16 is close to setting a record.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.