From Halopedia, the Halo wiki
Sniping is the act of covertly eliminating targets from a concealed or distant position. Snipers often rely on stealth and nearby cover; sniping is rarely performed from an open or conspicuous location. The most popular sniping weapons are the Sniper Rifle, Beam Rifles, and Binary Rifle. Others weapons (such as the Vostu-pattern carbine and the Battle Rifle) can also be used, though they are significantly less powerful and have a shorter range.
Sniping is an essential battlefield tactic in any form of combat. When correctly executed, sniping can inflict not only kills, but also a sense of demoralization and disarray among your enemy combatants. In The Art of War by Sun Tzu, it is stated that one must "attack an enemy when they are disorganized." Sniping can disorganize an enemy, and in a team game, this can make an attack much easier.
Sniping is effective both in free-for-all and team-based gametypes. If a sniper can find a good location to snipe from in a free-for-all Slayer match, then they can score a large amount of kills before other players are able to counter-snipe. It is entirely possible, however, for a sniper's opponents to temporarily cooperate in an effort to eliminate the sniper. Should a sniper need to retreat or relocate during a free-for-all match, they will not have allies to provide covering fire for them. Because of this, stealth is particularly important when sniping in a free-for-all match.
During a team-based gametype, snipers are extremely valuable. A sniper can potentially keep an entire team pinned down in one spot, and can also limit that team's exits and therefore their offensive and defensive capabilities. In objective-based gametypes, it may be very useful for the enemy team to know where a sniper is—this limits the team's options, making their actions more predictable.
Aside from their role as a useful distraction, snipers in team-based games can also provide very useful covering fire for teammates. Snipers can also act as effective scouts, locating hazards such as enemy vehicles. For these reasons, distance is particularly important when sniping in a team-based match.
When aiming for kills, a sniper's goal is nearly always to score a headshot. A headshot, when scored with a Sniper Rifle or Beam Rifle, is instantly fatal (unless the opponent has some form of additional protection, such as an Overshield). Multiple consecutive headshots from Carbines and Battle Rifles are not instantly fatal, but will bring a target down faster. In reality and in Halo, a sniper's motto is "one shot, one kill."
However, all headshot-capable weapons (even the magnum) kill in just one headshot, provided that the opponent's shields are down, or the shot is powerful enough to penetrate the enemy's shields.
The Sniper Rifle is the one of the three main sniping weapons. A single headshot can kill any opponent—even a shielded one (with the exception of Brutes on Halo 3: ODST) – and only one body shot is needed to kill an unshielded opponent. The Sniper Rifle has a slower rate of fire than the Beam Rifle, and needs to reload every four rounds; however, it can hold more rounds.
The Covenant Beam Rifle is the other main sniping weapon. Beam Rifles are roughly as powerful as Sniper Rifles. They are slightly more powerful, being able to kill Sentinels on Isolation in a single shot, but the difference is negligible when dealing with enemy players. The Beam Rifle has a slightly higher rate of fire and does not need to reload, but is prone to overheating and has fewer rounds.
The Focus rifle is a scoped beam weapon capable of continuous fire, being able to kill a fully shielded Spartan in about 1.5 seconds. It fires a continuous purple-tinted beam, but overheats easily due to the continuous fire. It is, however, incapable of performing headshots on other players or enemies.
The Binary Rifle is a scoped beam weapon used by Forerunner military forces, capable of killing with a single shot regardless of where it hits, making it more powerful than the Sniper Rifles and Beam Rifles. The Halo 4 version fires single shots that deal extreme damage with a downside of extreme accuracy loss when hipfired. The Halo 5: Guardians rendition of the weapon fires concentrated beams of energy that are fatal if they hit for most of the entire time they are fired, but lack the accuracy penalty when hipfired.
Neither the Vostu-pattern carbine and Mosa-pattern carbine, the Battle Rifle, nor the M'tara-pattern focus rifle are considered sniping weapons, but they can be used as such. Both weapons are headshot-capable, and both possess scopes that allow for long-range battles.
Like the Covenant Carbine, the LightRifle is not considered a sniping weapon, but can be used as such. It is capable of headshots and possesses a scope that facilitates long-range battles. The Halo 4 rendition of the LightRifle fires in three-round bursts when hipfired and powerful single rounds when scoped in, while the Halo 5: Guardians rendition always fires single shots but fires wider, more powerful rounds when scoped in.
This weapon was not designed as a sniper, but due to its high accuracy, power, and range, it can be used for sniping in a similar way as the Battle Rifle.
Like the Carbine, the Battle Rifle is not explicitly intended for sniping. However, it is scoped and headshot-capable, able to bring down a fully-shielded player with four headshots.
Like the Carbine, the DMR is not explicitly intended for sniping, but is scoped and headshot-capable, able to bring down a fully-shielded player with four headshots.
The Rocket Launchers have a scope, but are fairly poor weapons for sniping. The rockets that they fire are painfully slow, visually obvious, and generally easy to avoid. Sniping with a Rocket Launcher is only feasible when performed by a player with an incredible gift at leading targets, and against a player whose line of sight precludes them from seeing the rocket.
The Spartan Laser, introduced in Halo 3, has a relatively long range. Its scope is between the Battle Rifle's 2× zoom and the Sniper Rifle's 5× zoom, and it is certainly capable of killing an opponent in one shot. It has the disadvantage of being highly noticeable; it emits a red tracking laser for four seconds before firing, and its profile is rather conspicuous when being held or when being carried on a player's back.
The Incineration Cannon, introduced in Halo 4, has a relatively long range. It behaves much like the Rocket Launcher, but the projectiles not only move much faster, but are also capable of much more damage, as they leave behind explosive submunitions upon impact. The Halo 5: Guardians version of the weapon is one of the least ideal choices for sniping, due to its greatly reduced effective range.
Snipers are particularly useful when keeping a team coordinated. A sniper with a headset can report back to his team, relaying information on the locations of enemies, enemy vehicles, what weapons an enemy team is using, and other hazards. A sniper with a good position can survey the entire battlefield, searching for important events and observing them in detail with a scope.
When acting as a sniper, keep moving—especially in a free-for-all or Slayer game. In a free-for-all game, snipers don't have any allies that will provide covering or supporting fire; once a sniper is found, they have almost no hope of making it to safety without being seen or followed. This problem is compounded by the fact that many sniping weapons, particularly the Sniper Rifle, leave a bullet trail—a player, when killed, will know where the bullet came from. Frequent relocation can keep a sniper from being spotted and counterattacked.
In a Team Slayer match, a stealthy sniper is also important. If the enemy team knows the sniper's location, they can simply avoid that sniper. If the sniper's location is unknown, opponents may walk directly into the sniper's line of sight, giving the sniper an easy kill. A hidden sniper can also make the opposing team cautious—sometimes excessively so—and that caution can prevent the team from being as aggressive, decreasing the number of kills they will end up scoring.
A sniper should, when they begin sniping, already know where they plan to go next, and how they will get there. They should also have plans for a retreat, should their next location become inaccessible.
Leading in Halo: Combat Evolved
Halo: Combat Evolved's netcode requires players to lead their targets even more than in later Halo games. Players may often have to lead by a large amount to compensate for lag, which greatly steepens the learning curve for sniping in Halo: CE, more so than in the other games.
A sniper's secondary weapon should be a close- to mid-range weapon with reasonable firepower, such as a Carbine, Scattershot, Shotgun, M6D, SMG, Assault Rifle or Battle Rifle. Melee weapons, such as the Energy Sword, also work reasonably well. Extremely powerful weapons, such as the Rocket Launcher, should be avoided though.
The purpose of a sniper's secondary weapon is to dispatch nearby enemies. The secondary weapon should always be fully reloaded, and should be something that a sniper can safely use without risking an accidental suicide. (This is why weapons like the Rocket Launcher and Incineration Cannon should be avoided.)
The choice of a secondary weapon mainly relies on what the player wants to use it for. A player who is confident in staying far away from the enemy could use a Battle Rifle, Covenant Carbine, or even another Sniper Rifle or Beam Rifle.
Two players, one a vehicle pilot and the other a sniper, can cooperate to score kills from the air. The pilot must be skilled with a Type-26/54 Banshee or competent with a AV-14 Hornet; the sniper will have to be very good with no-scoping if a Hornet is used. Either way, pilot-sniper communication and cooperation is critical. The sniper can jump onto the Banshee's wing, or hop onto the Hornet's side seat.
If using a Banshee, the pilot must be able to dodge fire from Spartan Lasers, Rocket Launchers, and Missile Pods—all while ensuring that the sniper does not fall off of the vehicle. They must also be able to set up shots for the sniper and inform the sniper of any planned maneuvers. The sniper must in turn call out targets for the pilot, as the sniper often won't be able to take out heavily-armored targets (like Scorpions).
This tactic is highly conspicuous; a slow-moving Banshee is extremely suspicious, given that the aircraft is almost always used for high-speed aerial support. If the sniper and the pilot aren't skilled enough, they may be giving an opponent a free Double Kill. This is compounded by the fact that the Banshee cannot fly at too much of a vertical angle, and they cannot boost or perform a barrel roll; these actions will fling the sniper off of the vehicle.
A particularly useful technique is to have the sniper grab Active Camo before jumping into the Banshee; they can get covert kills, and then jump to the ground (or be dropped off at a high sniping position) to snipe normally.
There are a variety of anti-sniper techniques, ranging from tips to avoid getting sniped to strategies for counter-attacking a sniper.
There are a variety of ways to avoid being sniped or headshot.
In a team game, it can be particularly useful for some or all of a team to flank an enemy sniper—especially if the sniper's position, and therefore their line of sight, is known. The team must simply split up, going in opposite directions, staying at the edge of the sniper's movement, and exploiting every piece of cover they can find. Should they reach the sniper at the same time, they can form up and launch a coordinated counter-attack.
Alternatively, a single teammate can sneak away from the group, while the remaining teammates remain trapped by the sniper. The solo teammate can sneak around the edge of the battlefield and get to the sniper. If the sniper is not being adequately defended by their teammates, and is not skilled at close-quarters battle, then killing them should be a trivial matter (and may require nothing more than a simple assassination). Of particular note is the fact that a sniper using their rifle's zoom cannot see their Motion tracker.
At the same time, a sniper that has practiced battling at close range can be an extremely formidable opponent. Such snipers can usually no-scope a nearby opponent with ease; they also tend to watch their backs when alone.
When counter-sniping, find some cover and hide behind it. Aim so that if you were to jump, your reticule would pass over the enemy sniper—that is, aim slightly below where the sniper would be if you could see them from your present position. Zoom in, and jump. When the enemy becomes visible, fire.
This will likely reduce your accuracy, but it is also quite safe; you are only vulnerable while you take the shot.
Bring Out The Big Guns
When all else fails, use heavy weapons fire to blast the sniper apart.
A sniper can dodge shots fired from another sniper rather easily; all that is required is strafing. Dodging the splash damage from a Scorpion's blast is more difficult. Type-26 and Type-58 Wraiths are similarly dangerous to a sniper. A Gauss or Rocket Warthog can quickly eliminate an enemy sniper as well. Heavy vehicles are particularly effective if their pilots can approach the sniper without being seen until they fire the shot; if they fire from a shorter distance, the sniper will have a shorter time to see the shot, react, and dodge. Best of all, another tried and true tactic is to simply run your opponent over, though this has caused mixed results depending on which vehicle is used.