Well, you can't use all of the LMGs the same as each other. IMO the LSAT and MK48 act similarly to each other, with the MK48 being somewhat better IMO. The HAMR and QBB are much different. Here's my suggested class for them.
MK48 with Target Finder, Stock, and Suppressor Hardline or Lightweight (which ever you want) Toughness Engineer EMP x1 Primary Gunfighter
LSAT with Target Finder and Stock Hardline and Lightweight Toughness Engineer EMP x2 Perk 1 Greed
QBB with Quickdraw and Stock Hardline and Lightweight Toughness Engineer EMP x2 Perk 1 Greed
HAMR with Stock and Quickdraw Hardline and Lightweight Toughness Engineer EMP x2 Perk 1 Greed.
IMO, if you get scorestreaks somewhat often, Hardline is a good choice because it just makes it easier and you cycle through them much more often. Lightweight is kinda obvious...makes you a little faster considering you're pretty slow with a LMG. Toughness is a must IMO, because a lot of the time you'll be fighting at fairly long ranges, and landing all of your shots is good. Especially with the LSAT and MK48 because of the Target Finder...they suffer from very low recoil when using that. Speaking of the MK48, suppressor is a good idea because that guy has a pretty long 3 hit kill range. The suppressor doesn't do much to lower that. As for the third perks, it's whatever you want but I just like Engineer a lot.
A note on the QBB and HAMR, use them like high capacity assault rifles. The HAMR is kinda weird in that the first 5 or so shots have a higher rate of fire than the following shots. So if you can land most of that initial burst on someone, they'll die pretty quickly. Also, I don't use a pistol or anything with LMGs. You have a lot of bullets and shouldn't need another weapon. When needing to reload, find a quiet spot and do it. The LSAT and MK48 take forever to reload, but the HAMR and QBB are fairly quick...though have 25 less rounds per mag.
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Stock and either QuickDraw or red dot to have a clear view.
Mk42 is the best IMO, followed by the lsat, hamr, then qbb.
Also, I find getting headshots with them and racking up the kills for bloodthirsty is pretty easy, compared to the AR's. On almost all the lmg's I was able to get my bloodthirsties before completing the no perk no attachment challenges.
I've found the MK48 to be the most effective LMG. Low recoil, high power, and the range on it is massive so you're almost always killing in 3 bullets. I find the QBB and HAMR are both good, definitely a little better up close due to a higher fire rate, but overall less effective. The LSAT is IMO the weakest of the LMGs, kind of trying to be a jack of all trades but without enough strengths to merit the use of.
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