Well the lower tier support weapons aren't that good. Once I hit tier 3, I started improving but I had the Pro version of that weapon. The QKR (5th tier) was when it finally started to click and the Shred (6th tier) is pretty awesome. The damage and range is right up there with the assault weapons, and the huge clip size really helps against weak to mid level opponents with less than perfect aim, since they often have to reload while I can keep firing away with my 210 bullet clips. I generally use it at short to medium range and only ADS for long range. It also helps that I have the first perk unlocked, which starts regen (at a higher rate than usual) immediately following a kill.
Yeah, I was able to fly through the weapon upgrades because I got the bonus experience when I updated for veteran status, and some additional bonus experience in one of the supply crates I got with the 200 in currency they gave out. Once I hit the tier 3/4, it became a lot easier to put up respectable numbers in MP so I didn't ever have to grind in SP for experience.
Really enjoying it actually! It's nice to have something different to play as, and the larger clip size is awesome. I don't quite see the point of the one that has 30 bullets per clip though. That one caught me out loads as I had got used to not reloading, and then I would run out of bullets and get killed
