Guns, Lots of Guns
Tools of the trade, instruments of destruction, the essence of armed combat: Guns. Without a firearm, you're pretty useless on the battlefields of Ghost Recon. Fortunately, there are a lot to choose from. Unfortunately, it can be a little tough to pick 'em sometimes. Thus, I have decided to put my accumulated knowledge of these electronic replicas down on cyber-paper.

Assault Rifles
Automatic rifles are arguably the most effective all round weapons. They have power, range and versability. Riflemen with assault rifles usually make up the bulk of the traditional infantry squad.
Carbines
The smaller cousins of the assault rifles. The carbines are shorter and more manouvreable, but use rifle cartridges like the assault rifles. This makes them easy to use and very deadly.
Light Machine Guns
LMGs are mainly used for fire support and suppressive fire. They hold massive amounts of high-calibre ammunition, they're large and heavy and not too accurate. Use them to mow down groups of bad guys, or to keep said bad guys pinned down behind cover.
Sub Machine Guns
SMGs are defined as shoulder-fired weapons using pistol cartridges. They are very accurate and manouvreable, but the rounds also have more trouble with punching through heavier forms of body armour. Designed with close quarter combat in mind, they aren't very effective over longer distances.
Sniper Rifles
When the target's far away and you don't want to risk getting shot or detected, this is what you need. With high-calibre rifle cartridges and high-powered scopes, these babies grants you the capability to sit safely in a bush high up and blow holes through hostiles a couple of hundred metres away. Not very manouvreable, but very accurate once you get settled in.
Pistols
Back-up sidearms for those times when you run out of ammunition, or have to clear out a building and that sniper rifle just won't do the trick. Hardly the most effective weapons you can get your hands on, but they can still save your butt in a pinch.

M16A2
Everybody's heard of the M16. Tried and true, the M16 is the main battle rifle of several armies and special operations groups.
Despite the short zoom, it's quite decent, accurate at long distances and easy to wield. It comes with semi-automatic and three round-burst trigger groups, which will save you some ammunition, but full-auto fans will find themselves wishing for something more.
Single-Player: All the standard Rifleman grunts carry this one, as well as specialist "Buzz" Gordon. The zoom is sufficient, as are the trigger groups. Keep it on semi and double-tap the bad guys. It's manouvreable enough to use even in CQB situations.
Multi-Player: It works, on some maps. If you can deal with the short zoom, it's accurate enough to use for very long-range shots. Still, it feels somewhat weak and inversatile compared to the other assault rifles.

OICW
The Objective Individual Combat Weapon - the assault rifle of the future, some might say. This is basically a beefed-up M16A2. It has more zoom and a full-auto option, in addition to semi-automatic and three round-burst. For additional zoom, don't use the 20mm grenade launcher attachment.
Single-Player: This is only available through two of the specialists, Will Jacobs and Lindy Cohen. It's a formidable rifle with a very impressive zoom and range. Quite accurate and powerful, one might almost call it a full-auto sniper rifle. Use it to take down the enemy from afar.
Multi-Player: While it lacks the manouvreability of a true CQB weapon, setting it to full-auto still makes it lethal at close ranges. Otherwise, this is mainly a long-range weapon. Zoom in and blow them away before they even see you.

M4
Pretty much a shortened M16. It's capable of either semi- or full-auto firing modes, it's manouvreable and an excellent short-range weapon.
Single-Player: Standard Demo Experts carry these. They'll do the trick - the zoom is short like the M16, but it's fairly accurate. Good for both long- and short-range engagements, it's a versatile weapon, good for nearly all missions.
Multi-Player: Use this when you need a small, light weapon with good stopping power for CQB. Set to semi-automatic, it can be used for longer ranges in a pinch, but the short zoom will make it a tad difficult compared to some of the other weapons.

SA80
Like the M4, the SA80 is easy to use and with plenty of stopping power. That, however, is where the similarities end. The SA80 has a larger zoom and less recoil, making it a very good weapon.
Single-Player: Specialist Nigel Tunney packs this one, and thanks to that he's pretty much turned into a Rifleman-slash-Demo Expert. It has surprisingly little recoil, allowing you to effortlessly mow down groups of baddies using full-auto mode. It has long range and good zoom, making it good for long-range shots as well.
Multi-Player: Many players swear by the SA80, and it is indeed quite good. This is an assaulter's weapon, with a fast aim and little recoil. CQB or long-range, it works for both. A superb weapon.

M249 SAW
Short zoom, not too accurate, but with a whopping 200 rounds of ammunition to spray around and about.
Single-Player: As a support weapon, the M249 is carried by the standard Support Experts. Open up on hostiles and watch them flee for cover and keep their heads down, allowing your teammembers to get into better positions. It's ideal for taking down larger groups of hostiles, though only at close range. It is possible to do long-range shots with it as well by clicking quickly and firing only one or two rounds at a time. A.I.-controlled operatives with high 'Firearms' ratings will also be able to mow down resistance with little effort.
Multi-Player: Pretty much useless unless you're battling on small maps, such as the multi-player only ones. Since human players realize it's fairly hard to hit anything when going full-auto, thanks to the recoil, suppressive fire probably won't work quite as well as with A.I. enemies. Fire in bursts and try to be as close as possible when opening up.

MG3
A souped-up M249 with more powerful rounds, but only 100 rounds to play around with at a time.
Single-Player: Specialist Dieter Munz hauls this fast-firing cannon around. Other than the lower ammunition count and more terrifying sound, it's not really very different from the M249.
Multi-Player: A very good CQB weapon, and the loud sound is bound to give someone the heebie-jeebies every now and then. Use it the same way as the M249, at short range and in short bursts.

RPK74
Easily the weakest of the LMGs, with only 75 rounds in the magazine and barely able to hit a barn wall at twenty metres.
Single-Player: Specialist Guram Osadze carries the RPK74 and makes a decent back-up Support Expert if you need one. It does have one thing the other LMGs lack, however: a semi-automatic mode. Use this for long-range shots, then move in for the kill. Note that you'll have to move in even closer with this one, due to the lower accuracy.
Multi-Player: Every weapon is lethal in the right hands, and this is no exception. The addition of semi-automatic capabilities might possibly make it more versatile, and it certainly is capable of quite long shots. At very close quarters it's just as good as the other LMGs, and should be used in the same manner.

MP5
Small, light, easy to use, but with oh-so-little stopping power. At longer ranges, that is. In CQB, this thing is deadly, with just a couple of shots doing the trick in most cases.
Single-Player: Specialist Klaus Henkel uses the MP5. It's good for CQB, but not much else. It has the accuracy to make long-range shots, but the zoom and power negates it.
Multi-Player: If the map is small, this will easily do the trick. Get in close and in their faces, then spray them full. Avoid using it at large maps, as it will take quite a few rounds before the enemy goes down, and in that time they can return fire with a more powerful weapon.

MP5SD
Same as the MP5, but with an integrated suppressor.
Single-Player: Used by specialists Henry Ramirez and Susan Grey, it comes in handy for missions where you'd rather not get detected too easily. Thanks to the suppressor, it's easier to use at longer ranges - it'll still take several shots, but since they can't hear the shots, they won't be able to home in on you as quickly.
Multi-Player: The ultimate weapon for those stealthy players who prefer to sneak around and shoot others in the back. The MP5SD is completely inaudiable, and the only way to figure out where the shots are coming from is by using the Threat Indicator. This will help you create confusion among the enemy troops, as they see their comrades go down, but they have no gunfire to calculate your position from. Was their teammates shot from that bush, or perhaps the cliff, or was it that window over there? Mainly a CQB weapon, it can - like in single-player - be used for long-range shots with less risk.

M24
Small magazine capacity, short zoom and lack of stopping power makes this rifle somewhat poor. It's bolt-action, which slows it down a little.
Single-Player: Used by the standard Sniper Experts, it'll do the trick for the first few missions. Fire from a prone or crouched position and aim for the head.
Multi-Player: Sniper rifles are hard to master and seldom used online. Hardly a weapon to use while on the move, your best bet is to conceal yourself in a bush or similar. The fact that the rifle itself is camouflaged might help a little.

M82A1
The Barrett 'Light Fifty' is a huge rifle, and as an anti-material rifle it's really intended for use against vehicles and structures, not people (in fact, anti-personnel use is in violation of the Genéva Convention). Easily the most powerful rifle available, it'll punch through targets with ease. Semi-automatic.
Single-Player: Specialist Scott Ibrahim's weapon-of-choice, it comes with a powerful zoom that'll let you make out the eye colour of the enemies, and loaded with the powerful .50 calibre cartridge, it'll punch through combat boots, helmets, and everything in-between. Get into a stable position far, far away - and start picking 'em off.
Multi-Player: Perfect for setting up defensive positions on large maps, especially when overlooking a bottleneck. Stay put and hit them hard from afar.

L96A1
The second bolt-action rifle, but fortunately a lot better than the M24. It has ten rounds per magazine and it's quite accurate. The zoom is good, though no match for the M82 and SVD.
Single-Player: Brought into 'The Ghosts' by specialist Jack Stone early on, this will quickly become a replacement for the M24. The better zoom will be sufficient for a while, as will the power. It's bolt-action, so make sure they go down on the first shot - otherwise you might not get the chance for a follow-up shot.
Multi-Player: It doesn't have the best zoom around, but you'll still be able to hit from a fair distance. As with the other sniper rifles, stay put and stay concealed.

SVD
The forth and final sniper rifle is a semi-automatic with a good zoom and lots of stopping power. While no match for the M82, it does have less recoil, making it easier to maintain fire.
Single-Player: Specialist Astra Galinsky uses the SVD to good effect. It's surprisingly similar to the M82, so for the sake of simplicity, use it in the same way.
Multi-Player: This, too, is perfect for covering bottlenecks and the like while on the defense. Good zoom and power to boot. Less recoil makes this easier to use against multiple targets.

M9
The Beretta M9 - a not-too-powerful sidearm which you'll hopefully never have to resort to unless in 'Pistol Only' games.
Single-Player: If you have a primary weapon with ammunition left, chances are you won't be needing this one. Exceptions are snipers, whose rifles won't cut it when they have to get into the fray and clear a building or get rid of hostiles that have come too close for comfort. Aim and shoot, not much more to it. Just be prepared to spend quite a few rounds, unless you're good at headshots.
Multi-Player: If you run out of ammunition with your primary weapon and don't have time to reload, it might be a good time to whip out the M9. Otherwise it should be safely tucked away in the holster at all times. If you like to take chances, however, you could attempt to use your sidearm if you've managed to successfully sneak up on an enemy. Then you may proceed to taunt said enemy for as long as you like, assuming you kill him with the pistol.

M9 SD
Not much difference here, except for the addition of a suppressor.
Single-Player: Same as the un-suppressed M9, use for dire cases where you're out of ammunition, or for clearing buildings and close baddies with a sniper. You can also use it if your primary is un-suppressed and you don't feel like waking up the neighbourhood "just yet".
Multi-Player: No difference here either, use it when you're out of ammunition with no time for a reload, or when sneaking up on people. When sneaking up on someone, firing the M9 SD from a concealed position will make it difficult for them to return fire accurately, and will most likely cause them to panic.