"Just point and shoot." That's what all of the macho movie characters say as they hand loaded firearms to their pretty female counterparts. That's bullshit, and anyone who has used a firearm knows it is. If you give someone a firearm and expect them to be fine, there's a number of things that are going to go wrong. The first of these things is improper assessment of recoil. More likely than not, they'll assume the kick is going to be a lot stronger than it is, and they'll clutch the grip for dear life, ultimately pulling the gun off-target and missing by something like four inches. They'll repeat this mistake until they've familiarized themselves with the way the weapon handles, something they probably should have done long before an actual situation arose. The second issue that is almost guaranteed is inability to properly reload the weapon and make it safe when the situation calls for it, let alone clear a jam. This is a problem that even experienced shooters have when switching to a different weapon, so the idea that someone handed a weapon and expected to sort themselves out would do anything better than nip their fingertips in the slide and shoot them self in the kneecap is baffling.
It's not easy. It takes practice. And yes, you will be expected to properly roleplay all of these facts. You will need to roleplay cleaning your weapons regularly, or, instead, roleplay them misfiring. So read up, and take this information to heart, or admins will intervene and 'keep it real' (quite literally).
Gunshots are Fucking Loud: The Tale of Loud Fucking Gunshots
Gunshots are massively loud. Seriously. It's kind of unsettling how loud they are in comparison to the things we hear in everyday life. I've seen very few people properly roleplay how one would react on a battlefield without ear protection. While they might be too concerned with keeping themselves alive to bother with covering their ears, but they'd be borderline deaf by the time they made it out. Considering that the resistance hardly ever wears ear protection, an unexpected shot should warrant some reactions, as well as some post-action 'I can only hear two thirds of what you're saying' RP. Just remember that firing a round is not a private business; a gunshot will easily be heard anywhere on the map, though it may be faint. It is usually sure to attract the attention of an SCN unit, if not an armed Ground Unit.
Individual Weapons; Center-fire Ballistic Munitions
Center-fire ballistic munitions are the most common type of human-made munitions. Chances are, if you've fired a gun, it's been center-fire (or rimfire). Pretty much every weapon in Half Life 2 with the exception of Combine-made pulse weapons is center-fire. However, their similarities are minor, and each weapon has its own quirks, benefits and shortfalls.
H&K USP-M (9x19mm)
The USP-M is very misrepresented in the way that it appears in Half Life 2. In the game it is a relatively weak weapon, dealing minor damage and having somewhat unremarkable accuracy. In reality, the USP-M is a target pistol, and, in some cases, a race gun. The weighted recoil compensator on its muzzle keeps the weapon from recoiling upwards after each shot, meaning that keeping the gun on-target for quick firing is very easy. In real life it would be extremely expensive to purchase one, which is why they are typically only found in the hands of top-notch target shooters. 9x19mm is a suitably powerful round, usually failing to kill outright with a single shot, but certainly of ample power to take anyone out of the fight (see medical section for details). All in all, it's a very good gun, and to cast it aside for something else would be a shame.
Colt Python (.357 Magnum, .38 Special)
The Colt Python is a great gun. I can say this with some confidence because I've fired one. I was able to get a very good grouping and it handled quite well. However, using a revolver has its advantages and disadvantages. One of the biggest advantages is its complete inability to jam. The 100% manual operation means that there's simply nothing to catch. A problem that revolvers have is their hard trigger pull unless you pull the hammer back every time. Even if you do use it single-action only, the tiny six round cylinder means it takes a lot longer to load than to fire. All around, it's really not worth the trouble unless you absolutely need a larger caliber, and .357 isn't even all that large. Sure, it'll go through drywall like air, and it'll punch a hole through some soft body armor, but try using it against a plate-carrier vest or really any top-notch armor and you won't do much better than a semiautomatic pistol would have. There's a place and a time for the Python, but revolvers just don't have much of a place in modern combat, especially when most of your enemies are able to place multiple accurate shots downrange without having to adjust their aim much (See USP-M).
H&K MP7A1 (4.6x30mm)
The H&K MP7A1 PDW (inaccurately labeled an SMG by Valve staff) is easily the most adaptable weapon on the list. It can be held and fired as one would use a pistol if the grip and stock are collapsed, or it can be held like an SMG or rifle if you use the larger flip-up sights and extend the stock. It is very small and concealable, able to be carried in a thigh-holster or slung if need be. It's an extremely light and ergonomic weapon, but there's one thing that might put someone off. The round the MP7A1 fires, the 4.6x30mm round, is similar in appearance to a miniature rifle round. This has advantages and disadvantages. One of the key advantages is that the smaller powder charge is more than enough to propel the tiny slug at very high velocities, but small enough to keep the recoil and noise down. The tiny slug, since it is so angular and high-velocity, can go through most soft body armors (almost all types of Kevlar suitable for non-military use, including that which the MPF and Rebels use). However, the bullet is also small enough to have very little stopping power. This means that, while the long term damage may be fatal, a person might be on his feet and fighting after sustaining a hit to the limbs or torso. Sawing the tips off of your rounds or using hollow-point bullets helps with this, but it's not really a game-changer.
Franchi SPAS-12 (12 Gauge)
The main thing that distinguishes the SPAS-12 from other semiautomatic shotguns is its lack of a stock (as it appears in Half Life 2) and its ability to switch between semiautomatic and pump-action. It is important to remember that you do NOT need to cycle the action every time you fire the weapon if the selector is set to semiautomatic. The main reason the pump-action function is there is so that you can fire special loads (X-REP Taser rounds and other things with abnormal shell-lengths) without worrying about jams. Shotguns in general are very situational weapons. They are especially useful for door breaching, as a pistol cannot actually get a slug all the way through a padlock and knock it open, and a rifle will just blow a tiny hole clean through it and seal the lock permanently, but a shotgun will blow the lock apart. They are also very efficient against small numbers of enemies in close quarters. Contrary to popular belief, a shot to the torso will not instantly kill someone, buy it will be very, very hard to treat. Leaving a single piece of shot in the wound can be fatal, and it's very difficult to get every piece out without X-Raying the victim, a nicety which most people don't have in these times. Their spread is not as significant as games and films make it appear to be; you can easily hit an area target at a very good distance, and you certainly need to aim the weapon, even at close range.
Pulse Weapons
Pulse weapons all basically work the same way, and they have advantages and disadvantages vs. conventional ranged weapons. One of the primary advantages is that they can practically ignore traditional body armor. That superheated jet of near-plasma tungsten will go through kevlar like a lawn dart through a dog. This means that they are extremely modular, and the same general concept can be tweaked only slightly to serve different combat roles (Designated marksman rifles, assault weapons, squad support weapons, heavy gun emplacements etc.). However, using such a projectile also has many shortcomings. One of these is that, due to the round's superheated nature, it will usually seal wounds as soon as it makes them, preventing external bleeding entirely. This also means that there is no slug to surgically remove from the wound, so if the injury does not kill outright, bio-gel treatments, stitches and bandages are really all you need to treat a pulse wound (over a long time, I might add).
Treatment
Obviously wounds aren't going to heal on their own (no, you can't duck behind a car and wipe jelly out of your eyes, you buffoon!). You're going to need to treat wounds in both the short term and the long term. Remember, though, there are certainly other ways to handle this situation; I'm simply giving the way I would deal with it. Feel free to be creative, but be careful. I'll be describing what you should do if another person has been hit, since self-treating is typically beyond people. In fact, I'm going to go ahead and say you're not going to be able to reliably self-treat. Seriously. You'll kill yourself. In-Character and in real life.
Don't do it. In fact, don't try any of this in real life. Anyway...
How to stop someone from dying on the spot
Lets say your rebel buddy gets shot. At this point, he is out of the fight. There's pretty much no way around that, so don't try to wing it. If you get shot, you are out of the fight. The first thing you should do is dope him up to make sure he doesn't tear up your handiwork and kill himself. Give him whatever sort of anesthetic you have on hand (alcohol will have to do if it's all you've got). Once he's suitably loopy, plug the wound with cotton and wrap it with bandages (similar materials will do, but keep in mind what you're using), and tell him to keep pressure on it. Now you can deal with whoever shot him, if someone else hasn't already, and he won't bleed to death.
How to stop someone from slowly dying of infection afterwards
Once you've got your buddy out of harms way, remove the bandages you've applied and immediately sterilize the wound. Seriously, a dirty wound will kill someone fast, especially something deep like a gunshot wound. There's a reason your mother always made you put Neosporin on your cuts (if she didn't, you have a dysfunctional family). Once you've sterilized it, use whatever is on hand (clean) to get the slug out, as well as any fragments, but god help you if it has fragmented. Once you've finished doing that, go ahead and sterilize it again, just to be safe. Once more, plug up the wound with cotton and wrap it with bandages. Once this is complete, remind the victim to keep pressure on it for a while, and let it heal. (The healing process takes weeks, if not months. He's not going to be back on his feet the following day.)
Things you're not going to be able to treat
You can't save everybody. Seriously, there are lots of instances in which your patient is one hundred percent fucked unless you can use Combine technology to save them. One of these types of wounds is a wound to the lung or lungs. Once you take a bullet to the lung, your lung will flood over and over again with your own blood. You'll cough it up over and over, and eventually you'll pass out from air loss and die of internal bleeding. Unless you've got the technology to perform very invasive and very quick surgery, the patient is probably dead, and, considering the state of the resistance, you're probably not going to have enough time or enough tech. The same goes for most wounds to internal organs. A bullet to the intestines is going to fill your chest cavity with a mix of your blood, feces and bile. Again, you're probably not coming back from that, and it's not a fun way to go.
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Well, it's been fun, but this concludes my guide to Half Life 2's projectile weapons. I hope you've found this useful, and that you'll remember it while you RP.