From Halopedia, the Halo wiki
Halo: Combat Evolved
In Halo: Combat Evolved, Snipers is a game where there are only M6D magnums and Sniper Rifles. It takes 15 kills to win, the respawn time growth is on, there are no motion sensors, and the only vehicles are Warthogs. The Sniper is best suited for people who are very calm. In a snipers match, do not go into the open field, as you are easy pickings. At the beginning of a sniper match, try to find a place that is not on one of the main sniper perches. If you choose a commonly known sniper perch, everyone will look there first. Instead, try to find an out-of-the-way spot that you can see a lot of the map from. Go on and snipe. Of course, you can always use pistols in close-medium range combat. The Snipers gametype is also usually best experienced when on a large map.
Team Snipers became a very popular gametype when Bungie finally made it into a Ranked matchmaking variant for Halo 2. This was due to the incorporation of online multiplayer, which Halo: CE lacked. Some maps often featured in Team Snipers matchmaking are Sanctuary, Lockout, Beaver Creek, Relic and Turf.
As stated before, in Halo 3 Matchmaking, sniper-based variants are hard to come upon. Luckily, Team Snipers was also a Double EXP Weekend gametype on Halo 3. The variants in Team Snipers included: Team Snipers, Shotty Snipers and Team Splasers; all Slayer based games. Team Snipers is a traditional sniper game with sniper rifle, as the primary weapon and Magnum, which was the secondary firearm. Both of these weapons are set as default weapons. Hence, its title, there were no other weapons on the map that the player could use (i.e., battle rifle, assault rifle, rockets, spartan lasers, etc., in Team Snipers and Shotty Snipers). Nothing else, besides these two factors, were different from a standard Halo 3 Team Slayer game. Shotty Snipers, as the name implies, is just Team Snipers with a shotgun replacing the Magnum. Team Splasers is a little different from the first two variants, the respawn weapons being M6 Spartan Laser and Magnum. Bungie meant it to be Spartan Laser and sniper rifle, thus it is believed that a glitch is preventing the player from respawning with the Spartan Laser and sniper rifle.
Vehicles are in the variants of Team Snipers and Shotty Snipers. However, the only variant of the vehicles provided is the Mongoose. Some experienced players will use these light armored vehicles to its full potential, especially in Shotty Snipers, as the passenger in the back can use their shotgun to kill enemies at close combat. This hit-and-run method is not recommended, however, due to the lack of armor cover the Mongoose provide and should be avoided in Team Snipers.