Like some other Half-Life mods, and unlike early (Betas 1-3.56) versions of AHL, V1 onwards has incorporated Lag Compensation. This means that you don't need to lead your targets (except with knives, of course), because the server checks to see if your target was in your crosshair when you fired, and if they were, they will take damage. If you have a medium or high latency (150-300), this manifests as people dying half a second after you shot them. If you are used to leading your targets in online games, this may take some adjustment. Just pretend that you're in Single Player mode again, and you should be fine.
Combat in AHL has many features that you may be unfamiliar with. Many things affect your accuracy. First, if you run around with a sub-machine in full auto, jumping all over the place, don't expect to get too many headshots. Movement states in order of most to least accurate are: lying prone, ducking, standing, walking, and running. Jumping or just being in the air also decreases accuracy. Furthermore, your weapon mode and rate of fire come into account. Firing a weapon in auto fire is less accurate than semi-auto fire (if these modes are applicable). Firing in short bursts is also more accurate than simply holding down the button. Finally, if you have a laser sight and your current weapon can be fitted with one, your accuracy is given a significant boost.
AHL also has locational damage. This means that if you get hit in the head, you take a lot of damage, and will often die instantly. A shot in the arm will take off less damage. A shot in the chest or stomach will deal medium damage. Leg shots deal half damage. Getting shot in the leg will also cause you to 'limp' until you bandage your wound. You will also limp if you fall from a great height and take damage. Note that the shotgun and handcannon do not do locational damage, meaning no headshots and no leg shots.
Bleeding is another aspect of AHL combat. Whenever you take damage from a weapon, you will bleed. If left untreated, this wound will slowly cause a lot of damage. To prevent this from happening, you can bandage your wounds. Unlike some other mods, you have an unlimited supply of bandages. Just press 'B' to stop the bleeding. Take cover before you bandage, however, as you will be unable to fire a weapon until you have finished bandaging. You will also be restricted to walking while you are bandaging. Furthermore, you leave a blood trail whenever you are bleeding. This can be a good way to track down a wounded enemy, but it's also a good way to get tracked down yourself.
Unlike regular Half Life and many other mods, all reloading is done manually. This means that you must press the Reload button (R by default) when you run out of bullets. With the Shotgun and Colt SAA, you have to hold down the reload button while all the shots are loaded. This allows you to control when you reload, and how many shots to put in the Shotgun and Colt SAA. Often, you won't want to take the time to fill those two all the way up. Reloading any weapon will take a second or two, so try to take cover first. It is usually faster to switch to another weapon than to reload your gun. If you have a holster, weapon changes are instant, and switching weapons when you run out of ammo is a very good idea.
There is another thing to keep in mind when reloading a weapon that uses clips. If you reload before your gun is completely empty, there will still be a round in the chamber, so after you insert the next clip, you will have an extra bullet, or two extra bullets with akimbos! For example, The Desert Eagle starts with Seven rounds in the gun. If you fire off all but 2, then reload, you will end up with Eight bullets in the gun.
All weapons except knives and berettas have some form of penetration. This means that they can be fired through walls and doors. The better a weapon's penetration is, the thicker the wall can be. If a bullet successfully penetrates a wall, it will hurt anyone on the other side. Many heavier weapons have good penetration. This means that the bullets will go through walls, doors, and Kevlar vests, and hurt people on the other side. Keep this in mind when taking cover from a guy with a rifle or Magnum.
Whenever you run around in AHL, you make footstep noises. Most enemy players will be able to hear you coming if you are close, and you will lose the element of surprise. There are two ways to mask your footsteps. First, you can 'walk' instead of run, by holding down the walk button or crouching while moving. Walking or crouching cuts the volume of your footsteps in half. Second, you can choose Stealth Slippers as your item. This will remove all of your footsteps, no matter how fast you're moving.
Whenever you use a stunt, you lose a small amount of stamina. Generally you will never notice the stamina system, but if you run out of stamina by stunting repeatedly, there will be a brief period where you cannot stunt at all, while your stamina regenerates. This system is primarily to prevent people from repeatedly diving all the way across a map.
In-Game voice communication was added to Half-Life in the 220.127.116.11 patch. AHL incorporates these features in Beta 5. Setup and use of the voice comms is identical to TFC, however there are some rules regarding who you can hear and when. In Deathmatch, you can hear everyone all the time. In Teamplay, you can only hear living teammates when you are alive. In Last Man Standing, you can only hear living people when you are alive. When you are dead, you hear everyone, living and dead. Note, you can mute people by clicking on their name in the scoreboard.