You get M14 at level 46, but level 37 (I think) is the G36c, which is a very deadly weapon. Especially with the red dot site or just 'spray and pray' at point blank range!
The 50 cal. has a good rate of fire and can shoot through a lot of stuff even without deep impact. Give it stopping power + deep impact and it becomes unstoppable....
I've never been any good with the G3 on multiplayer, but it has THE best hipshot on single player so I'm good with it there.