When is an MG-42 a medium MG and when is it a heavy MG?

[D-Day_Dodger]

I have read that generally speaking the German MG-42 machine gun is considered a medium machine gun, however when mounted on its quadpod it is rated as a heavy machine gun. What I'm curious about is how does the stability of the gun platform affect it's hitting power? It seems to me that velocity is a matter of how much explosive force is in the cartridge. Things like the stability of the gun platform and rifling should affect accuracy but not penetration. What am I missing here?

Another ballistics question that occurs to me is why does the longer the barrel equate with greater accuracy? One would think that with a rifled barrel, regardless of length, once the spin is imparted to the bullet it will go in a straight line to its target. But this is obviously not the case. I'd sure appreciate a layman's explanation.