So you want something that shoots both semi and full auto.
I'd say get an M4 and stick an electric winding box mag on it, it looks silly but it's the most effective lmg you can get..
PROS:
-light (depending on what gun you actually get)
-lots of bbs to throw at the bad guys
-full of upgrades/replacements
CONS:
-looks silly
-may not be allowed at milsims since it's not technically a support weapon..
I do have a 600rnd electric magazine for my m4s that I use when I need to lay down massive amounts of fire, and a guy in my team has a 2500rnd c-mag for the same reason.. But that looks even more stupid on his short m4 (the mag is as large as the gun's length lol)..