I'm getting the ARs diamond atm, from what I've used so far I'm a big fan of the MTAR, M27 and Scar. Acog + Stock is my preferred loadout for ARs.
Select Fire Fal is good, better in Core than HC IMO. Not a fan of Select Fire on the SMR though, far too slow. Better off spamming the trigger!
I got them diamond so a lot of work with them all. The Fal nd the SmR are beast if u have a fast trigger finger or with select fire, but not my fav ones. Here's my goto class:
Type 25 w/ silencer, extended mag and stock.
Perk 1 ghost
Perk 2 scan
Perk 3 ( either ) tac mask or eng. (depeneds what score streak I'm using like care package)
Now also the type 25 is the fastest shooting AR too So u can take off the stock and use laser works pretty well. Or use a red dot site. That could help quite abit.
I use to enjoy the type 25, and the swat. But now that I have the m8a1 I never use anything else.
If you put the select fire, target finder and another attacment it seems to work well. For short range I recommend the vector with the long barrel and I like target finder for my site.
My fav is the Type25 with Stock and Supressor.
I use it with Lightweight perk and it behaves a little like a high powered SMG. It's the only weapon I have gold camo on. The problem is the recoil is a bit erratic because it has quite a high fire rate but one thing that helped for me is trying the Reflex sight.
Close 2nd is M27. I use this as my 'engineer' class with Engineer perk and Black Hat with Scvenger. Usually with a supressor and stock attachment.
Unfortunately the range of both of these ARs is pretty bad. That's why I don't mind using Supressor seeing as the range is the weak point. Try removing the Supressor if you need a bit more range.
If you want an out and out AR with a lower rate of fire but great range and damage (basically a more realistic AR), try the SCAR-H, or the MTAR until you unlock SCAR.