Some thoughts after I went running with rifles for a couple of hours.

The Assault Rifles: AK47, G36
Looking at the data file confirmed my suspicions that these were very similar, in fact identical. These weapons are obviously pretty well rounded. Generally a good choice. Continuous fire with these weapons is not the best idea, unless you've got cover you can reload behind. I can't really think of much to say about these weapons.

The Light Machine Gun: M240
Because of the prone requirement these are a pain to use. However, if you can lay down on a hill or if you've got a wall to shoot from, you can shoot pretty well. This makes it great for defending, but difficult to use on offense. I usually won't go out of my way to pick it up, but if you spawn with it you can put it to good use. Unlike the other weapons, you probably shouldn't ever release the trigger; for you team players out there, your task with this weapon is to lay down suppressing fire, and this weapon does it well. Just reload when you run out of ammo and start shooting again. Don't even get up. It's not as exciting as the other weapons, but it can be really powerful.

The Suppressed Sub-machine Gun: MP5SD
Suppression is great. You could just sneak into an enemy base and start picking people off, because they don't react very intelligently to comrades dying, or even streams of bullets flying by their face. Even if that's not your thing, it's good for sniping because you can unleash a flurry of bullets without your foe being any the wiser. It may be my imagination, but it seems like it's also good for flanking distracted enemies; I think the suppression keeps them from realizing they're being shot at from another angle. So what I do, I follow an attacking squad off from the side, and then when that squad gets in a firefight with the enemy, I'm at a good angle to pick off enemies also without attracting too much attention to myself. This strategy might work just as well with an assault rifle, but I've never tried it. In a straight firefight, an assault rifle is probably better, but this has a higher rate of fire which can be a life saver when you just need to kill someone before they kill you. Spray and pray is actually an okay strategy for this weapon because of the rate of fire and spread, but you still shouldn't be caught reloading in the open.

Other opinions welcome, obviously.

I made a very unfinished page on the wiki about weapons.
http://runningwithrifles.wikia.com/wiki/Weapons
Would be great if you could help out and add some information.

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Oasis. Less Running, more rifle.

Actually, I've been able to use the m240 to great effect offensively.

Just find a nice wall on one end of an enemy base, fire some rounds towards a group of them, and they advance to kill you.
The good spot and huge mags makes for a slaughter.

I know I'm posting this video again and again, but it's a good example of using the M240 offensively: http://www.youtube.com/watch?v=GYla3ZCwCeE

My only issue with using the m240 offensively is how easy it is to get flanked. Would be a good tactic to bring friends.

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Oasis. Less Running, more rifle.

If you watch yourself, keep a bearing on where the enemy is coming from, and don't advance when you shouldn't, flanking isn't too much of a problem.

Especially now with more mobile forces, you can get reinforced if you plan it right.