I haven't used them in BO2 but the best LMG based on the stats is the MK48, it has the largest 3 hit kill range but it has the slowest reload speed.
The second best is the LSAT but it to has a slow reload speed.
"Right now Von Miller is the best pass rusher in the world. He looks exactly like Derrick Thomas did in his heyday."
- Jon Gruden