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
Let me start by saying the Spas 12 has the most power and largest spread pattern, but high recoil, slow rate of fire, and slow load time. Now the XM26-LSS is the only Full Auto Shotgun, but only holds 6 rounds. It uses a clip so it has the fastest reload time. It has the weakest Damage, but at close quarters is very deadly. Unfortunately most player matches have restrictions that don't allow you to use this weapon. So I suggest the 500 Tactical. It has a smaller spread pattern than the Spas 12 but has a higher rate of fire and packs more punch if you get someone in the small spread. One shot kill weapon!