[GentleKiller]

MG 15 - single machine gun, old design, low ROF.
MG 81 - Much newer machine gun, 1500 rpm or so.
MG 81Z - Two of them bundled together.

So you get the idea. An MG 81Z had the rate of fire of some miniguns nowadays.
Much more devastating if you manage to hit.
EDIT: Much more chance to hit, too, actually

is 81z on me110g2?