I run a M60 E4, the thing is a beast. Upgrades are almost a must, i tossed in a top speed ratio gear set and a tm eg1000 motor into mine the thing just spews bbs.
Generally when I see my local bastard I buy ten or fifteen bags at a time to be sure i have enough bbs for a while, in a 4 or 5 hour game I generally onl go through about 2 bags or 4000 rounds. I generally keep one boxmag full plus two reloads on me just incase though there has been the odd game that I have used more than that.
SAWs are damn heavy though if you get a full metal one, and not one of the wussy plastic ones my 60 is almost 20lbs when its loaded. So be sure to get a well padded strap.
The three things im generally tasked to doing are assault, ambush and defence
assault and defence are self explanitory. What lost people overlook is what a saw can do in an ambush situation.
When ambushing an enemy get some good consealment, a ghullie suit helps alot, pelreferably near a trail. And just wait for some enemys to pass by you dont want to start shooting untill there about 30 feet away and open up. If you dont wait you might not get them all and have fire coming back your way. This works really well if you have a fire team with you.