Particle Beam Rifles are most often found being used by Jackal Snipers, or in Covenant weapons holders. In Halo 3, they are rare on the lower difficulties.
When using the Particle Beam Rifle, it should be remembered that it is a sniper weapon, and should be used at long ranges, to take out high priority targets (preferably) with headshots, or in the case of Hunters, a shot to the exposed orange flesh. Due to its difficulty to use at close and medium quarters, it is advisable to keep a good short-range secondary weapon, in case an enemy moves too close to use the Beam Rifle effectively.
Jackal snipers on Heroic and Legendary can be extremely accurate and can kill you in one shot, regardless of where it hits, making them probably the most lethal enemy in the game. Remember where there are snipers and eliminate them first.
The particle beam rifle is ineffective against the Flood as the shots go right through them doing little damage. You can kill combat forms by hitting them in the chest area (where the Pod infector's tendrils protrude, but the target is fairly small). In addition, you can use the beam rifle to hit carrier forms from a safe distance.
When the Tilt skull is on, the Beam Rifle is unaffected by it unlike its Human counterpart.
On the level, The Great Journey, Sergeant Johnson uses the Beam Rifle to lower Tartarus' shield. The player can also obtain a Beam Rifle before the battle and use that to lower the Chieftain's shield instead of waiting for Johnson to do it, but Tartarus' shield recharges more quickly if the player uses the latter method. A better method would be to give it to one of the Elites that helps you kill him, that way, you have two times the shots, but his shields won't recharge immediately.
In Halo 2, the Particle Beam is featured in quite a few of the large multiplayer maps such as HRUNTING/YGGDRASIL Mark II (J) Colossus and Relic. In Multiplayer, getting the Beam Rifle should be a priority. Its long range and lethal headshot capabilities offer a huge advantage whether you are on the offense or defense. Picking up Active Camouflage will allow you to snipe enemies while remaining stealthy. Remember to keep a good close range weapon such as the M90 Shotgun so that you will be able to fight enemies too close to kill with the Beam Rifle. A medium range weapon, like the Covenant Carbine, could also come in handy in case of a close range fight.
In Halo 3 multiplayer, the Beam Rifle is featured on maps like Snowbound and other map variants that support the beam rifle. It has a limit of 10 shots as it drains slightly more battery units due to balance issues. The Beam Rifle can also be used to fire two shots in quick succession, although it will overheat immediately. This tactic can be used to quickly dispatch enemies at close range. A popular tactic is to camp and kill any enemy who attempts to obtain a power weapon by watching it with the sniper rifle. This keeps control of the more powerful weapons in the hands of the player watching the weapon.