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 MG36 and MK46 both carry 101 rounds. The MG36 is more like and Assault Rifle, has low kick and a lot of power. I suggest this weapon out of all the Light Machine Guns. It is also the most accurate LMG.