What brand is your M249? You could get a really solid and fast ROF from an 11.1v 20c lipo. You just have to be sure your internals can take the beating.
Check this website out. They have lots of good batteries for under $10. I know lots of people who have used the Zippy brand lipos and have liked them a lot.
I have two King Arms 11.1 15C 1100mah batteries. I would get something different, but these are the only batteries that fit in my current gun. Still, they are awesome and they fit into ANY gun.
1800 rounds a minute is a stupidly high ROF. Putting that many rounds down range isn't going to keep any more heads down, just make you go through BBs faster and can cause a malfunction of your GB that much faster.