100-200 rounds is extremely fine for most cover fire contact break situations.

Also, several of the belt fed uppers will take standard magazines, so you just drop the belt pouch when it's empty and reload with regular mags until you can drop another belt in.

SAW gunners do this all the time, and, from what I've seen, some of these uppers are much more reliable than the M249 which is the crappy standard LMG in the US arsenal for the past few decades.