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

[Steel God]

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?

Strictly speaking, the MG42 was never a "HMG". It was used as a LMG when fired on it's bi-pod, and as a MMG when fired on it's tri-pod, but to pass to the category of an HMG it usually involved upgrading the ammunition to something heavier than the rifle rounds the MG42 used. When referred to as an HMG, the MG42 is really just well sited, with established fields of fire, and a crew serving it to keep it supplied with ammo and fresh barrels. The MG42 was an awesome MMG and LMG, but never a true HMG because of it's ammo type.

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.

From wikipedia: http://en.wikipedia.org/wiki/Muzzle_velocity

A gun's muzzle velocity is the speed at which the projectile leaves the muzzle of the gun. Muzzle velocities range from subsonic (below 330 m/s / ~1080 ft/s) for some pistols to more than 1,800 m/s (~5910 ft/s) for tank guns firing kinetic energy penetrator ammunition. The latter velocity is close to the limit achievable with chemical propellants.

In conventional guns using gunpowder, muzzle velocity is determined by the quality (burn speed, expansion) and quantity of the propellant, the mass of the projectile, and the length of the barrel. A slower burning propellant needs a longer barrel to burn completely, but can on the other hand use a heavier projectile. A faster burning propellant may accelerate a lighter projectile to higher speeds if the same amount of propellant is used. In a gun, the pressure resulting from the combustion process is a limiting factor on projectile velocity. A balance between propellant quality and quantity, projectile mass and barrel length must be found if both safety and optimal performance is to be achieved.

The velocity of a projectile is highest at the muzzle and drops off steadily due to air resistance. Generally, smaller diameter projectiles lose velocity less rapidly than wider ones. While a lighter projectile will move faster, it may not carry more kinetic energy and if physically smaller, must carry a smaller internal explosive charge.

Longer barrels give the powder more time to work on propelling the bullet. For this reason longer barrels generally provide higher velocities, everything else being equal.