This is a solution that I am writing for each of the Kill 100 enemies with a specific weapon class achievements. I am currently working on these achievement myself, so the following solutions are what has worked best for me. My current rank is a Major, mostly earned through player matches, so I have been playing for some time now. Alright here are the weapons classes, followed by a detailed summary of the specific class.
Pistols: Glock 18c or Desert Eagle
Sub-Machine Guns: P90 or Type 05
Shotguns: XM26-LSS or Tactical 500
Assualt Rifles: FAMAS or 552 Commando
Light Machine Gun: MG36 or MK46
Sniper Rifle: PSG1 or SR-25 SD
The P90 and the Type 05 are the only two Sub's that carry a 50 round clip. Considering it takes about 4 to 5 bullets to the body to kill someone, you'll need the 50 round clips. The P90 is probably one of the most accurate Subs, has a very fast rate of fire, and very little kick. The Type 05 is more like an Assault Rifle. Has a very slow rate of fire as compared to other Sub's but packs more damage and penetration. Depending on your playing style, I would choose between these two weapons.