For that purpose, just develop reliable drum mags for the regular AR/AK platform.
If volume of fire is a concern and you don't have the top hatch of the vehicle available or don't want to poke out of it for whatever reason, ARs and AKs can function as an adhoc LMG in a pinch.
If you HAVE to put a heavy volume of fire down, dismount. There's very few reasons you'd need a LMG just to keep firing from the windows of the vehicle. Either move or dismount.
Lastly, the thing with LMGs or intermediate cartridges in general is, they need that lengthy barrel for the powder to burn completely and get as much velocity for the round as possible. It also helps with cooling. A longer barrel heats up slower and cools faster (thanks to the surface area of the metal).
They tried that "smaller LMG" concept with the sawed off RPD in Vietnam and it was very niche. It could be used, sure, but as soon as you went beyond cities and into typical engagement ranges (which, in Vietnam, were still not that far. >200 meters usually), it would lose a lot of accuracy.
There's always a compromise. Longer and bulkier the gun, better the range and accuracy. You compromise one, you lose the other.
Another problem is belt fed systems are naturally bulky. Even if you put a small barrel on it, the way the cartridge gets picked and ejected after firing and the way it eats the belt up requires a very large receiver and lots of parts moving within.
Do you have a particular MG system in mind?
(Sorry for talking like ChatGPT).