For typical LMG long range supression and area denial, Mk 48 has no equal. FMJ is the one mandatory attachment at all times, cover will mean nothing. Then use and combination of Dual Band (my choice), Target Finder, Suppressor and Rapidfire. You will still kill in 3 hits over every sight line in the game, but RF makes you fare more deadly in mid and close distances. Plus, you burst fire at long range anyway for the 2hit head shot.
For all purpose and more standard death match gameplay, LSAT is most well rounded. Great mag size, great irons, good fire rate, good range and good penetration. If you want to do everything well, pick this. Stock and Quickdraw are all you need, but suppressor is a good choice too, as is FMJ.
And for the bulldog bullet hose its the QBB LSW. This one is great for hardpoint and headquarters. Laser Sight gives you great hipfire, and suppressor eliminates muzzle flash. Stock and Quickdraw help also.
You should run Toughness on everything. Mk 48 needs hardline because youll pick off so many enemies from afar youll rack up high streaks easily. This makes the FHJ a great companion for extra points, and use shock charges to protect your rear and alert you to enemies. The other two benefit from Flack Jacket and Lightweight combined.
You have too many attacments on the mk 48 recommendations. Besides which, I really dont notice any difference in bullet penetration with fmj on most maps.
That was meant to be "any" combination of those attachments. On the long maps, FMJ retains much of your 3 hit kills even through walls. On the smaller maps, you can get by without a sight and stick to Rapidfire/Silencer/FMJ or just two of them, but the Mk is meant to be played long anyway. I always use Dual Band and FMJ myself and alter nate between suppressor, rapid fire, or no third at all.