As a startup weapon I'd recommend the PDW-57 (Sub Machine Gun) more than anything, and if your style is more long range then a Light Machine Gun or learning how to use Snipers early can be a big benefit.
I used the MTAR at the start and found it was increasingly underpowered at short and long range as the game went on. I've always just been more of a SMG kinda guy, though I would recommend the dreaded combination of LMG with targetfinder if you're learning how to play.