1) The ammo restriction for a SAW depends on the ammo restrictions for the rest of the players. If you're running realcaps for everyone, same goes for the SAW.
2) Individual players may carry a 'belt' of ammo for the saw, ranging between one to two hundred rounds for a realcap event, adjusted accordingly for non-realcap events. Can be carried in bags or 35mm film canisters.
These are probably the only restrictions you'll need, and I'll explain why:
1) Reloading a 249 with a box mag or highcaps is going to take JUST about as long as if you were to reload a real one. You'll have to be down for a reasonable amount of time while you open the mag, fill it, and close it back up.
2) An individual carrying a 249 or M60 will suffer the same accuracy problems when standing and firing as those who are operating the real thing. Those who are more capable of accurately firing the SAW while standing due to physical stature should not be penalized and made to lay flat.
3) While limiting the ammo to 200 / boxmag, it makes a saw almost useless as a squad support weapon if the individual only has 1-3 box mags. 600 rounds can be topped by any SINGLE M16 gunner carrying 20 mags. Allowing other individuals to carry 'extra ammo' is accurate and it makes the weapon more useful. ALSO encourages them to feed the gun in order to keep their asses safe.
"Solving an imaginary world's contrived and over dramatic problems... 6 millimeters at a time."