I don’t think the M250 is replacing the M240, it’s replacing the M249, though I could be wrong.
200 rounds of 5.56 in the SAWs nutsack is being replaced by 100 rounds in the M250 because the ammo is so heavy.
The MACVSOG guys were using 20 rounders because they couldn’t get 30 rounders.
Major Plaster details how some of the recon guys mail ordered 30 rounders from the US because they couldn’t get them through the supply system but needed them badly.
According to Tilt Meyer:
“I carried thirty-four 20-round magazines for the CAR-15 – we only placed 18 rounds in each magazine, which gave me 612 rounds for that weapon”
I don’t think 612 rounds of Sig Fury would be doable.
Many SOG guys didn’t carry a backup pistol, so they could hump a couple of more magazines. Weight of ammo matters a lot.