CheatyGlitcher

M249's would probably be easiest to source.

A HPA'd A&K can be brutal - someone had one the other week and it sounded like a baby A10 warthog was doing a gun run.

Other more pricey but arguably more realistic version are the G&P, TM MK46 or the Lambda MK48

The nationality of the vehicle would help , aren’t going to see an RPK on a UK/US themed one , same as you’d be unlikely to see a 249 on an OPFOR vehicle either ?

^as above

if the nation is a choice yet to be made then the pkm might not be the worst pick, with the aftermarket mag mech you can get 7k rounds on tap and the boxes are pretty quick to change out if you have multiples and are running the power input seperately (eg a massive battery somewhere in the vehicle, or build the box to run off the vehicles 12v system)

plus it gives you an excuse to buy a pickup truck