The only time I'd use an LMG is if I'm spawn trapping in Domination on Terminal. It's such a good combo with Claymores and FMJ. Pretty much guaranteed 4 kills+ if you have C and B.
The only time I use a Mounted Machine Gun is at the start of Wasteland on the A spawn. I pump it down to C and usually get 1-3 kills. Pretty nooby but a few people do it, so if you can't snipe 'em, join 'em