In all honesty, if you have a quick enough shot with the L96A1, you don't really need an AR/SM/MG. The sniper rifle can easily take out medium to long range targets (and if you're good enough close ranger). The only problem most people face is with targets within 30 yards due to the crazy bunny hopping they tend to do which makes getting off a sniper shot somewhat difficult. You could go with an assault rifle to take them out but that means less support items in your inventory. The load out I use when I snipe is..
Primary: L96A1 w/ S1 silencer and Extended Clip 1
Secondary: Military Shotgun (extremely over powered and imo is much better than having an automatic weapon in your backpack)
Support: Flash bang
Backpack: Frag/Mines/HBS (depends on what game mode i'm playing)
The shotgun is just crazy. 30 yard headshot with a shotgun is just wrong. Plus about 75% of the time its a 1shot kill (depending on range and where you hit them). If you don't kill them with your first hit, then most likely that second shot will finish them off easy. Also, especially in really close quarters, you'll want to bunny hop a lot against an automatic rifle which if you had an auto would throw off your accuracy. However the spray of the shotgun is barely affected at all by the bunny hopping so all you have to do is aim in the general direction and you should be fine. Having an extra slot in your backpack can give you a little more toys to play with.