creating a new, fictional oob

[PlasmaKrab]

do you maybe know i way, to model the plasma guns in a way, that their efficiency gets smaller with higher range to simulate the volatility (right word??) of plasma

The only way I know of that would allow to figure that effect would be by using the AP ammo type only. AFAIK only both AP and Sabot rounds will lose penetration over distance, while HE and HEAT are supposed to have some immoveable warhead in addition to mere kinetic energy.

That would be what forces your weapon to lose efficiency with greater range. The only drawback is that you can't use it against soft targets, including aircrafts.