This is the three-barreled gatling-gun you'll see on the back of the Warthog. Hop on up and man it to give the fellow driving a bit of teeth to go with his tail.
All in all, the M41 (usually called "the gun in back") is a fine weapon for its uses. Electrically-driven, it fires very fast, and due to the magic of video games, will never run out of ammo - or need to be reloaded, since it's chain-fed. It's mounted on a swiveling platform in the back of the Warthog with a 360-degree range of motion; if you can see it, you can face it (although it should be noted that its vertical movement is somewhat more limited; if a target is very close (hence, very low) or very high, you may not be able to aim at it). Better yet, it moves independent of the Warthog's motion; useful if you have a very bumpy driver.
The drawbacks of the "gat" are as expected; lousy accuracy being the main one. You can get decent long-range shooting if you fire in short bursts, but that utterly defeats the purpose. More likely, you'll end up simply holding down the trigger to take advantage of the infinite ammo, spraying it at nearby enemies as you drive past them. Tracers will help you stay on target. The damage of individual shots is only fair; but the gun fires so fast that it almost doesn't matter. If you can hold someone under your targeting reticle for a few moments, they should drop.
The other obvious disadvantage of the Warthog's gun is that the thing's bolted down. As much as you'd like to, you can't carry it with you. And no, you can't drive everywhere. And unfortunately, it would require two controllers, four hands, and multiple personalities in order to shoot and drive simultaneously; so you need either another human player (friends... remember those?) to help you out, or you'll end up in the "Drive - see enemies - stop Warthog - get out - man gun" situation, which can be a hassle. Which isn't to say it's not sometimes very nice; as a suppressive-fire device against large crowds, and especially against vehicles (Banshees are downed like butter), the gatling gun is a magic wand. But it's not perfect.
And no. There is no melee attack. Unless you count running people over, which is a function of the Warthog, not the gun. Go away.
Pros: Breathtaking rate of fire, mobile firing base, very good anti-vehicle and swarm-fighting abilities.
Cons: Inability to use without an accompanying Warthog and driver, minimal accuracy.
Usage: More handy as protection for the Warthog than as an offensive weapon. Get in on the action as your mate runs folks over. No longer fear Banshees and other heavy vehicles. But don't try to use it to snipe.
For: Your friend.