LMG's are kind of a strange bunch. They're effectiveness falls somewhere under the assault rifles and SMG's, but I can't say I ever really use them.
The main reason I don't is because they are so slow. Everything becomes slow with these guns: Sprinting, strafing, sighting, throwing grenades, and most
noticeably, reloading. The worst of these though are the sighting and strafing. Those are some of the most fundamental
aspects of how one-on-one encounters play out.
If you aim down the sights too slow or too late, you're dead. If you can't strafe well and
an enemy is
already firing at you, forget sighting with these things, you're
going to die. The hip-fire of LMG's are rivaled only by Sniper Rifles in terms of crappiness. You could fire about 20 - 30 rounds
standing 10 feet away and miss the guy.
They're true domain however lies in they're ability to provide covering fire for teammates, and that's about all they're good for. All in all, I would have to say stay away
from using these guns when you're serious about winning, they're ok, but Assault Rifles, SMG's, and even Shotguns would serve you better.
M249 SAW
Range: Infinite
Accuracy: High
TTK: 0.195s
Tier: 3
Well it might come as a bit of a shock to you, but the seemingly "weak"
SAW is actually not the weakest of the machine guns (take a look at the
TTK's). I've already mentioned all of the problems with these weapons so
I guess I'll just talk about the good things of each one, since they all
have some good aspects about them. The coolest thing about the SAW is
that when it is equipped with a Grip attachment, its hip-fire rating
actually becomes higher than that of an Assault Rifle, but lower than
that of an SMG. If you really want to try LMG's, but you're used to a
very aggressive run-and-gun style of play, I would recommend you
give the SAW a chance first since your hip-fire will still be decent. The
bad news is that the muzzle flash alone can really mess you up when
aiming down the sights.
RPD
Range: Infinite
Accuracy: Very High
TTK: 0.171s
Tier: 2
Among most LMG users, the RPD is probably the most popular, and taking a
look at the TTK value it's not hard to see why. With infinite range,
this thing has a flat rate TTK which is already pretty high, so combined
with a Grip for reduced recoil, it can be pretty effective when mowing
down groups of enemies. That being said it still suffers from the same
plague that all of the LMG's do: slooooooowwwwness. You'll be gunned
down at close-range for sure unless you get the jump on the guy and mid-range most of the time
you'll be beaten by SMG's
and AR's. I would put it in the first tier but since it's just to slow
to rely on all of the time.
Poor iron sights, horrendous recoil that will throw you off at nearly
any range, and the already poor hip-fire accuracy is made worse by its
incredibly slow rate of fire. And take a look at its TTK value, .200 -
.300?! That's such BS, it's perfectly on par with the MP44, which is
also a crappy gun but STILL beats this thing because of better hip-fire,
iron sights, and recoil. The ONE thing that this LMG does is make
Stopping Power somewhat unnecessary. The question then becomes what else
will you use? Double Tap is a bad choice because it makes your already
bad recoil worse, doubles the muzzle flash so you can see after you
start shooting, and doesn't get a big percentage boost in the first
place since the fire rate is so slow (compared to something like the
Mini-Uzi with DT). Juggernaut works best with it, but you'll still move
too slow for most one on one fights for it to matter. Sleight of Hand is
unnecessary when you could just use an AR. Going back to the sights,
they're so bad that you'll want to use a RDS or ACOG scope...But then
that prevents you from using a Grip! Do yourself a favor and pick
something else unless you don't care about winning or just really want
to act like Rambo.
M249 SAW 
RPD
M60E4