There's no right answer to this question. Everyone likes playing different classes, and most players have 2 or 3 classes they play regularly. When you're getting bored with a class, try a new one. The contrast in playing style between classes is one of TF's strengths.
There's a class for everything, and the trick is to find a class that's good at doing
what you want to do. If you want to play Offense and capture flags, playing a Heavy
Weapons Guy will only make things harder for you.
Here's a quick rundown of playing styles. Read over them, and pick the one that sounds the most fun to you:
Scout - The perfect flag capturer.
Scouts are the best flag capturers, so they are utterly essential in any map which involves a flag (maps with objectives like Capture the Flag or Territorial Control). Scouts can be effective even without firing a single shot, since their strength lies in their speed rather than in their combat skills. If you like moving twice as fast as everyone else, and screaming for an escort as you escape the enemy base with the Flag and the entire enemy team pursuing you, then the Scout is your class.
The Sniper is the only class that can kill an enemy on the other side of the map with almost no warning at all. Be careful though, for if the enemy engages you at close range you're weaker than most of the other classes. If you like settling down into a good sniper position and methodically killing every enemy who exits his base with a single shot to the head, then the Sniper is for you.
The workhorse in any TF team, soldiers perform well at any task and specialize in none. If you like killing lots and lots of enemies in straight man-to-man combat, this is the class for you. Soldiers are a core class in any offensive or defensive squad.
Carrying more explosive weaponry than any other class, the Demoman clears whole rooms in seconds. He's essential on maps where the use of high explosives can destroy barriers, allowing access to more entrances into the enemy base. His remotely-detonable bombs make him a vicious defender in enclosed spaces. If you like watching the smoke clear and seeing bodies everywhere, the Demoman's your class.
A solid close-to-medium range fighter, the Medic moves swiftly and carries a gun that enables him to mow down any of the weaker classes without too much trouble. His real strength lies in his ability to heal himself and his teammates with his medikit. If you love seeing eternal gratitude in the eyes of your teammates, or can't aim at all and still want to be incredibly useful to your Team, go for the Medic class. You might not kill a lot of enemies, but the teammates whose lives you've saved sure will.
The name says it all. A slow moving, heavily armored beast toting an enormous assault cannon, the HW Guy can mow down any enemy in seconds. Not a lot of finesse in this one, since the cannon spits out so much fire you barely need to aim it. If you like picking a defensive position in a map and saying "No-one, and I mean no-one, is getting past here", then the HW Guy's the class you want.
A great first line of defense, the Pyro can easily knock chunks out of the enemy with his flamethrower. He doesn't have much in the way of killing weaponry, but he can make sure no-one gets into the base without being set on fire, and being burned alive tends to upset people. If you like running into a room, firing wildly, and then watching enemies run around screaming as they roast, then the Pyro is perfect for you.
A class who can disguise himself to look like anyone, the Spy can often walk into the enemy base, right past the defenders. Able to kill an enemy in a single hit with his knife, the Spy is someone you definitely don't want to turn your back on. After a few of their members have been knifed in the back, teams tend to get fairly paranoid. You know you're winning when they start to shoot each other for fear they're Spies. If you like matching your wits with the enemy instead of your reflexes, then the Spy is your class.
When your base needs some heavy defense, the Engineer comes to the rescue, building automatic sentry guns that track enemies and kill them. Able to repair his teammates' armor and build machines to dispense ammunition, the Engineer is often the core player in large defensive squads. If you need to defend your base against all those hardcore players out there with much better aim, the Engineer's the perfect class.
If you've read over TF's player classes and you're still not sure which one to pick, choose one that suits your experience level. Medics, Soldiers and HW Guys are all good first-time classes. Once you've got the hang of one or two of them, move on to some more complex classes, like the Engineer and Spy if you want to avoid a lot of combat, or the Sniper, Demoman, and Pyro if you like being in the thick of things.
Every single enemy you meet is a better fighting class than you are. Remember, your goal is to get the Flag, not kill the enemy. Leave the killing to your teammates who are playing fighting classes.
Your enemies will be doing everything they can do to prevent you from getting into their base. Use your "Move Left" and "Move Right" keys to dodge incoming fire. Even if you can't see an enemy, keep dodging as you run, because chances are good that there's a sniper aiming at you.
If the enemy has a defensive position that you just can't get past, or a sentrygun that keeps killing you, call for backup. Tell your teammates where the enemy (or sentrygun) is, and let them take care of him for you.
If there are a lot of enemy defenders in a room, use your Concussion grenades to soften them. Hold down your "Throw Grenade Type 2" key for a couple of seconds before releasing it. This will throw the Concussion grenade a second or so before it goes off. Hopefully the defenders won't have time to avoid it with only a second's warning. After the grenade has exploded, run like the wind! Concussed defenders find it hard to hit any target, let alone a Scout running at full speed.
Caltrops are small metal spikes that injure and slow down anyone who walks over them. When you're being pursued, hit your "Throw Grenade Type 1" key to drop a canister of caltrops. Try and drop the canister just around a corner or in a narrow passage to ensure the enemy runs over the caltrops. If you're lucky, this will slow them down long enough for you to escape.
The sniper rifle takes awhile to master. The trick to using it is to learn to aim effectively. When you press your "Fire" button, you begin to aim. Your rifle will raise slightly and a red laser sight will appear where you're aiming. When you release the "Fire" button, the rifle fires. The longer you hold down the "Fire" button, the more damage the shot does. If you aim for a full 2 to 3 seconds before you fire the shot, you'll kill almost any class in a single hit. If the enemy is a fair distance away, use the zoom capability of the rifle by pressing your "Use Special Skill" key (right mouse button by default).
Always remember that the sniper rifle is a rifle, not a pistol or shotgun. You can't just shoot from the hip. If you're running, you won't be able to fire the sniper rifle until you slow down and take aim. If you need to fire on the run, use your nailgun or auto rifle.
The sniper rifle does more damage if you hit your target in the head. This damage adds to the damage created by the amount of time you aimed before firing. You'll kill anyone if you aim for a couple of seconds and hit them in the head. Another thing to note is that if you shoot them in the legs, you'll slow them down.
You're fairly weak when it comes to close combat, so don't let your enemies reach you. Find a position where they can't get to you easily. If they do get near you, don't try to kill them with the sniper rifle. Change to your nailgun or auto rifle and gun them down. You can try lobbing some grenades at them to keep them away too.
It's a good idea to start aiming before you see your enemies so that you can build up a bit of damage before you fire at them. Since your laser sight is visible, learn to hide it in places where your enemies can't see it.
The auto rifle is a great weapon for finishing off targets who you know are low on health. If you hit them with a well-aimed shot from your sniper rifle, it's more than likely they'll be low on health, so switch to your auto rifle and pump a few rounds into them. They'll have a hard time dodging you.
Your rocket launcher only holds 4 rockets at a time. When it's empty, you're vulnerable. You can reload at any time by hitting your "Use Special Skill" (right mouse button by default) or "Reload" (R by default) keys, so whenever you've got a moment throw a rocket or two into your launcher. Smart enemies will be counting the number of times you've fired, and they'll try and attack you when they think you're empty. Reload before you run out, and you can give them a nice surprise when they charge you.
When your throw a nail grenade, it rises into the air and then begins to spit nails in all directions for a few seconds before exploding. It's a great room clearer, and the perfect way to keep enemies pinned down. If they're pursuing you, drop one behind you as you run. If you're defending a room, you've been told enemies are incoming, and you need health, drop one into the room before you run to the Resupply Room.
One of the unique things a soldier can do is rocket jump. It's a term for a fairly brute force method of gaining some height. Aim your rocket launcher straight down at your feet, and press your "Fire" button and your "Jump" button at the same time. The rocket will hit the ground at your feet, exploding and propelling you up into the air. It'll hurt you a bit, but it's a great way to get into areas of the map that are normally inaccessible. It takes a bit of practice, but once you've got it you'll be the bane of snipers everywhere as you rocket jump into their sniping positions from the ground below.
Your main weapon is your grenade/pipebomb launcher. It has two modes, grenade and pipebomb. In grenade mode it lobs grenades that explode on contact with enemies. If the grenades don't hit anything they explode after a couple of seconds. In pipebomb mode, the launcher lobs pipebombs. These are simply grenades that don't explode on contact. Instead, you remotely detonate them at any time by hitting your "Use Special Skill" key. These are great for setting traps!
You can have too many pipebombs lying around at once. If you reach your limit, your oldest pipebomb will automatically explode when you lob another one. Pipebombs will explode after they've lain there for a couple of minutes, but you'll rarely have to worry about this. Also, your pipebombs are rigged to your heartbeat, so when you die, all your pipebombs explode. It's not uncommon to see them blow up the same guy who just killed you.
Your grenade/pipebomb launcher can only hold 6 grenades at a time, so when it's empty, you're in trouble. You can reload at any time by hitting your "Reload" key, so whenever you've got a moment, throw a few grenades into your launcher. Smart enemies will be counting the number of times you've fired, and they'll try and attack you when they think you're empty. Reload before you run out, and you can give them a nice surprise when they charge you.
No other class carries as powerful grenades as you do, let alone as many. Your MIRV grenades explode into smaller grenades which then explode again, killing anyone and everyone near them. These are great for clearing entire rooms. If you know there are enemies ahead, throw a few grenades in and watch them come running out.
All your weaponry is powerful at close range, but none of it is very useful at long range. Avoid having fights with enemy classes that are dangerous at long range, like the Sniper and Soldier. Either charge them, dodging as you go, or retreat behind cover and hope they come to you. Wherever possible, fight in enclosed areas where they can't avoid getting hit by the explosions from your grenades.
Some TF maps have specific obstructions that can only be destroyed by a detpack. These will be identified in the map Briefing screen. Use the Command Menu to set three different types of detpacks. Place a detpack in front of the obstruction by selecting Command Menu/set detpack/detpack type. It takes 4 seconds to place the detpack, during which time you'll be unable to move or shoot. If your enemies are smart, they're going to try and kill you during this time, so you might need to get a teammate to escort you. You can only carry 1 detpack at a time.
Pipebomb jumping is a crude method of gaining access to areas you can't get to normally. Throw a pipebomb out, then go and stand on it. Press your "Jump" key and your "Use Special Skill" key at the same time. This will cause you to jump and detonate the pipebomb simultaneously. The pipebomb's explosion will do a fair amount of damage to you, and propel you up into the air. It takes some practice, but once you've got it, you'll be able to get to places you couldn't before, like up into the middle of the enemy sniper positions.
When you look at your teammates (by pointing your crosshairs at them), the status bar in the lower left corner of your screen will show you their health and armor. If their health is less than 100%, they need some healing. Select your medikit by hitting your "Use Special Skill" key, run over to your teammate, aim your medikit at them (just like you would a crowbar), and press your "Fire" button. One "hit" and they'll be back to full health.
Your medikit will heal everything. If you see that one of your teammates is concussed, hallucinating, infected, crippled from a leg shot or caltrop, or burning from a pyro's flamethrower, use your medikit on them to cure them of their affliction. They'll be eternally grateful.
You can do even more than just heal and cure your teammates. If you use your medikit on a teammate who's already at full health, you'll inject him with adrenaline, increasing his health 5 points over his maximum health level. The maximum you can adrenalize him up to is 50 over his normal maximum health. Adrenaline is short-lived, and the player's health will slowly drop down until it's back at his normal maximum health. This is an excellent way to give an attack squad a boost before they enter the enemy base.
If you use your medikit on an enemy, you'll infect him with an infectious virus. It'll drain his health slowly, until he either dies or is healed by a Medic on his team. The virus is particularly lethal because it's airborne. If the infected player gets near any of his teammates, the virus will infect them as well.
Other teammates might signal to you that they need healing by shouting out "Medic!". You'll see a flashing red icon above the player who is calling for assistance. Heal 'em!
Don't go on Offense with this class until you're fairly experienced. You're the slowest moving class in the game, and you'll be picked off by enemy snipers before you even get into the enemy base. You're one of the strongest defensive classes in the game. Just keep firing your assault cannon at anyone firing at you, and they'll be the ones to die first.
Your assault cannon takes a second to wind up before it begins firing. This means that it's not a good idea to guard places where a Scout can appear and get past you before you've begun firing. Enclosed spaces are also bad for you because they allow the enemy to jump out, lob rockets and grenades at you, and then duck back behind cover before you can hit them. That said, large open spaces are also dangerous for you due to your slow speed. Snipers and Soldiers can pick you off from a distance, where the inaccurate fire of your assault cannon renders it fairly ineffective. Luckily, enemy snipers rarely make it into your base. An ideal defensive position for a HW Guy is at one end of a long corridor that enemies have to move through, allowing you to fire at them as they approach.
Scouts hate you. You're their worst nightmare. The only thing on an enemy Scout's mind when he sees a HW Guy on Defense is "How can I get past him?". Do your part to make it harder for Scouts by standing in doorways that they need to get through. Scouts will run up and attempt to squeeze by you. Ignore them if they're shooting at you, since they have about 1/5 of the armor you have. Wind up the assault cannon and let 'em have it. If you're feeling particularly nasty in a Capture The Flag map, go and stand right on top of your Flag. Watch out for their concussion grenades though.
Try and get an Engineer or Medic to defend near you, so that you can get them to heal you and repair your armor. Since you're so slow, it takes you longer than any other class to get to the nearest Resupply Room, stock up, and return to your position. If you've got a Medic and/or Engineer nearby to save you the trouble, you can ensure your position is never left undefended. Remember to use your "Call for Medic!" key to signal that you need assistance from them.
The strength of a Pyro comes in his ability to set enemies on fire with his flamethrower and incendiary cannon. The more you hit your enemies with fire, the longer they'll burn and the more damage they'll take. You're a great class for sowing confusion in the enemy ranks, mainly because it's really hard to aim effectively when you're burning to death. Keep in mind that your weaponry doesn't do a lot of damage though, and it'll take quite a while to kill armored classes like the Soldier and HW Guy. Get in close, set them on fire, and retreat while they burn. Keep doing it until they die.
The flamethrower is your main method of setting enemies on fire, but the incendiary cannon has two advantages that make it essential in some situations. First, and most obvious, is that it has a far greater range and can ignite multiple targets at once. The rockets travel fairly slowly, but have a larger blast radius than the Soldier's rockets. They don't do much damage, and they don't set everyone on fire, so they're great for lighting up the enemy Snipers and providing the confusion your team needs to get a squad across the open areas between bases. Secondly, and most importantly, they ignite enemies through walls. The heat generated by the rockets penetrates right through walls, floors and ceilings, setting any enemies on the other side alight as well. If the enemy Snipers are causing you trouble on 2fort, run under them and fire some shots into the ceiling.
One of the most useful things a Pyro can do is be the first line of Defense for a team. Guard the entrance to your base, and make it your mission to set every single enemy who passes you on fire. Don't worry too much about killing them, just make sure you ignite them a bit. Once they're past you, warn the rest of the team's Defense that there are incoming enemies. This early warning, combined with the fact that no attackers reach the main Defense without being singed, can often be the thing that prevents the enemy from breaking through.
Napalm grenades set an area of ground on fire for a few seconds, igniting any players who try to move through it. They're great for blocking off an area, saving time and probably your life. Drop them on the ground if you're being pursued, or throw one into rooms filled with enemy defenders to force them to move away from their positions.
The Spy menu appears at the bottom of the first Command Menu level. The following commands are available through the Spy menu:
Your number one skill as a Spy is your ability to look like a member of the enemy team. Using the Command Menu, select which team and class you would like to masquerade as. Wherever possible, avoid letting the enemy see you as a Spy. Once they know you're playing a Spy they'll be much more alert. Luckily, the paranoia induced in a Team from the knowledge that there is an enemy Spy wandering around is worth playing a Spy for in itself. It takes a couple of seconds to disguise yourself, so try not to be discovered.
The Spy is the thinker's class. You need to outwit your enemy. After your first couple of infiltrations, it's fairly likely the enemy is on the lookout for you. They'll be watching for anyone on their team who's doing anything out of the ordinary. You need to watch the enemy so you can become one of them. Pick your disguise carefully, and change it whenever you're alone and the disguise is no longer fitting. If the enemy has one Sniper, and he's defending the base entrance, don't disguise yourself as a Sniper and walk into their Flag room.
Remember that your teammates will also see your disguise. So if you're disguised as an enemy Sniper, your teammates will probably think you're an enemy Sniper too. If they're smart, they'll read your ID in their status bar and know you're a Spy, but usually, they'll pump a few rounds into you. You can help prevent this by sticking to a simple rule: Never change your Team disguise inside your own Team's base.
As a spy, attacking anyone is not a decision to be made lightly. This is because when you attack, you lose your disguise. That's right. You're back to looking like a Spy, and you can be sure that any enemies who see you won't be amused that you've been pretending to be their buddy for the past 2 minutes. The only exception to this rule is grenade throwing, which means that you can throw a grenade and still maintain your disguise. Unfortunately, any enemy who sees you throwing grenades around is probably going to get a little suspicious.
Enemy Spies and Scouts will remove your disguise if they touch you, so avoid them if you're disguised. Try not to look like you're avoiding them though.
Some maps have objects that remove your disguise. For example, in Capture The Flag maps, if you grab the enemy Flag while you're disguised, your disguise will be lost, and you won't be able to re-disguise while carrying it.
Use your ability to feign death a lot. Spys are very effective at killing specific individuals on the enemy team, so they are particularly lethal on maps where 1 or 2 classes are important. For example, in CanalZone 2, Scouts are essential because they're needed to carry Flags from their base out to points on the map. Find places where the Scouts will be moving past, and lie in wait for them. Be careful not to look around while you're "dead", because the enemy will be able to see your "corpse" moving. Wait until they pass you, then stand up and knife them in the back. Another useful thing to know is that you can disguise yourself while you're feigning death, which is often the safest way of disguising yourself.
When used right, your knife is the most lethal weapon you carry. If you stab an enemy in the back with it, it'll kill him in one hit. Even if your stab isn't perfect, it'll still inflict severe damage upon him. Bear in mind that as a result you'll lose your disguise, and you can be sure that if there are enemies around to notice this, they won't be amused.
If you're in the enemy base, waiting for the perfect opportunity to grab their Flag while no-one's looking, or to knife a particular Defender in the back as your Team's attack squad comes in, make yourself useful. Report back information to your teammates. Tell your Team where the enemy Defenders are located, what classes they are playing, where their sentryguns are, and so on.
Sentryguns are dumb. They won't fire at you if you're disguised as a teammate of the Engineer who built them. They won't even fire at you when you throw grenades at them. This makes you the ultimate anti-sentrygun weapon. If your team's Scouts are getting mowed down by enemy sentryguns, walk on into the enemy base and put some grenades on the sentrygun when no-one's looking.
The engineer's Build menu appears at the bottom of the first Command Menu level. The following commands are available through the Build menu:
The following options only appear if you've already built a sentry gun:
The following options only appear if you've already built a dispenser:
All your abilities as an Engineer rely upon you having enough metal. To get metal, grab armor from your Resupply Room. Any armor you can't wear is automatically converted to metal. To see how much metal you've got, select your Wrench and looking at the "ammo" count in the bottom right corner of your screen. 200 metal is the most you can carry. You need 130 metal to make or upgrade a sentrygun, and 100 to make a dispenser.
Your most useful contribution to the Team is your sentrygun. You can only have one of these built at a time, so positioning it well is important, although you can always dismantle it and build another. Don't worry if it quickly gets destroyed the first few times you build it. Just look for a new place to put it where the enemy will have a harder time killing it. Keep an eye out for other Engineers, and see where they're placing their sentryguns. Once you've picked a place to build yours, select Command Menu/build. If there's no option to build a sentrygun, that means you don't have enough metal, so go get some more from the nearest Resupply Room. It takes 4 seconds to build your sentrygun, during which time you won't be able to move or shoot, so if you're building outside your base you might need someone to guard you.
Once you've got your sentrygun built, you'll want to upgrade it quickly. Sentryguns have 3 levels, and at each increasing level they have more health, fire faster, and hold more ammo than at the previous level. Level 3 sentryguns fire rockets as well as their gattling guns, making them particularly vicious. To upgrade your sentrygun, make sure you've got at least 130 metal, and then hit your sentrygun with your wrench. If have 130 or greater metal your sentrygun will be automatically upgraded. If you don't have enough metal, go get some more from the nearest Resupply Room. Note that once your sentrygun is level 3, you won't be able to upgrade it any further.
Once the sentrygun's up and running and killing some enemies, you'll need to maintain it. If you look in the bottom left of your screen, you'll see your sentrygun's health and ammunition. If it's low on health, you'll need to repair it. Hit it with your wrench and select the "Repair" option from the menu. Keep doing this until it's fully repaired or you run out of metal (you can see the amount of metal you've got in the ammo count in the bottom right corner of your screen). If your sentrygun is low on ammunition, you need to restock it. Grab some shells from your Resupply Room, hit your sentrygun with the wrench and then select the "Put ammo in sentrygun" option. If it's a level 3 sentrygun, make sure you grab some rockets and put them in it. Keep doing this until it's fully stocked with ammunition. Finally, make sure your sentrygun's facing the right way. Sentryguns track better if they're facing the direction the enemies' are coming from, and they'll sometimes swing around and end up facing the wrong way after they've shot down an enemy. Select Command Menu/Build/Rotate Sentry Keep rotating it until you're happy with the direction it's facing.
The next thing that your team would be eternally grateful for is an ammunition & armor dispenser. This is a small machine that creates ammunition and armor inside itself every 10 seconds, and lets anyone get some from it. It's like a mini Resupply Room. If you've got a group of Defenders in a place where they can't get to a Resupply Room quickly enough, a dispenser's the perfect thing to allow them to resupply without leaving their posts. Dispensers are also great if they're hidden somewhere near the enemy base to allow your attack squads to resupply right before they go into the enemy base. Like sentryguns, you can only have one dispenser built at a time, so place it wisely. They have little health, so keep your dispenser fully repaired if you can. Hit it with your wrench to bring up the Maintenance menu, just as you do for the sentrygun.
Another important and often overlooked trait of the Engineer is his ability to repair his teammates' armor. Simply hit your teammates with your wrench. You'll need a bit of metal to repair them, but not much. Keep hitting them until their armor's fully repaired, or until you've run out of metal. Note that your teammates will probably use the "Call for a Medic!" command to ask you to repair their armor, so if you hear them yell for help, look around. The player who called for help will have a floating red cross above his head.
The other useful thing an Engineer can do is make ammunition. This is great for swiftly delivering ammunition to Defenders, like HW Guys, who don't have time to get to the Resupply Room and back to their posts before the enemy attacks. Making ammunition is easy. Simply select Command Menu/Discard Ammunition. Unlike every other class, who only drop the ammunition they're carrying that they can't use, Engineers always drop some ammunition of each type, using their metal supply to fabricate ammunition that they don't have. If you don't have much metal, you won't be able to make much, but its probably still a better idea than sending that HW Guy back to the Resupply Room.
Your EMP grenades are tricky. They explode like any grenade, but instead of doing any damage themselves, they detonate anything explosive around them and rely on that to do damage instead. This means players with explosive ammunition, like rockets and shells, will explode like firecrackers. Detpacks will go boom. Pipebombs will detonate. Ammo packs will go up in smoke. Roughly speaking, the more ammunition lying around, the more dangerous the EMP blast is going to be, and some classes, like the Soldier and HW Guy, are carrying a heckuvva lotta ammunition. The only downside is that some classes, like the Scout, are carrying almost no explosive ammunition at all, and can simply walk right through an EMP blast. Use your EMP grenades, combined with your hand grenades, to destroy enemies who are trying to destroy your sentrygun. Your sentrygun is far more dangerous to them than you are, so if you're lucky, you'll be able to throw a grenade at their feet while they're otherwise occupied.
Here's a breakdown of the grenades used by each class:
Grenade Type 1: Caltrops.
Grenade Type 2: Concussion grenades.
Grenade Type 1: Hand grenades.
Grenade Type 2: None.
Grenade Type 1: Hand grenades.
Grenade Type 2: Nail grenades.
Grenade Type 1: Hand grenades.
Grenade Type 2: MIRV grenades.
Grenade Type 1: Hand grenades.
Grenade Type 2: Concussion grenades.
Grenade Type 1: Hand grenades.
Grenade Type 2: MIRV grenades.
Grenade Type 1: Hand grenades.
Grenade Type 2: Napalm grenades.
Grenade Type 1: Hand grenades.
Grenade Type 2: Gas grenades.
Grenade Type 1: Hand grenades.
Grenade Type 2: EMP grenades.
If you simply hit one of your grenade keys, you'll Prime and throw the grenade immediately. This gives enemies a good three seconds to get clear of the grenade, which is usually enough time for them to make it to safety. To prevent this, press and hold your grenade key for a couple of seconds, and then release it. The grenade will go off a second later, which shouldn't give them enough time to react.
Players defending a position are only effective while they're at their post. If they have to run off to the nearest Resupply Room to get more ammunition, they're leaving their position undefended. You can help them out by discarding any unused ammunition you have whenever you run past them. Every second they can spend at their post and not resupplying is another second the enemy won't be able to get into your base.
Resupply Rooms contain ammunition for every class and weapon, but no classes actually use all four types of ammunition. It's a good practice to always discard your unused ammunition as you exit the Resupply Room, leaving anything you didn't need to the next player, who might need it.