In most games you need an actual support weapon (ie a gun thats a support weapon in real life, M249, M60, RPK, etc versus an M16 or AK47 with a drum mag) to be able to use it as a support gun; this is especially true here (Montreal/Quebec area) with real cap games becoming increasingly popular.
So what you're looking for is a support gun, which narrows down the search a lot. Just about all of them come with a box mag, that will hold 1000-3000 BBs and usually have some kind of motor -driven feeding mechanism. As with most AEGs, RoF will mostly be a function of your battery size. You may want to look into adding a MOSFET to your setup (for several reasons). Most support weapons come with a gearbox that can stand up to the function.
It comes down to what you like, how much you're looking to spend and how much work you want to put into it (some setups will require some converting, like some RPKs or an MG36).
And in response to Dustin's comment: I've carried an M249 for 10 hrs or so during a milsim. It IS heavy and tiring, but so's an M4 w/M203 etc. Honestly, in the heat with gear on, after a while anything gets heavy. Get a good sling. For me, the most annoying thing by far about LMGs is the box mags, which are big noisy maracas (specially when doing recon style stuff, which is what we do).
Last edited by Drake; March 19th, 2013 at 14:11..