The BR55 is the all around go to weapon when facing Covenant forces, as it can effectively take down Grunts and Jackals with one well placed three shot burst to the head (torso shots require about two to four bursts to kill).
Given that the bullets bounce straight off a Jackal's Energy Shield, fire a well-placed shot through the small notch on the shield's right, and, as the Jackal recoils from the wound, finish it off with a head shot.
Elites' energy shields are very resistant to the BR55. Try to keep the rifle leveled at its head and keep firing until it falls dead. It is advisable to soften an Elite up with grenades, melee attacks, or a Plasma Pistol's charged shot before engaging them with a BR55, especially on higher difficulties. On higher difficulties, the Plasma Pistol/Service Rifle Combo is extremely efficient, as it only requires one overcharge and a headshot against any Elite.
In Halo 2, you can kill a Brute with three head shots, as the first two bursts will knock off the helmet, and the following headshot will kill the Brute. However, shooting a Brute in any other area is not particularly effective due to their extreme resilience (on Heroic it takes about 17 3-round bursts, or one and a half magazines, to bring down a Brute).
When engaging a Hunter, dodge behind them, firing as many bursts as possible into their exposed orange sections before they turn around.
The BR55 is somewhat limited against the Flood, however, a 3-round burst to the center of a Combat Form's chest where the Pod infector is will generally destroy it.
A quick melee followed by a burst can dispose an Elite in seconds.
The BR55 replaces the M6D magnum from Halo: Combat Evolved. Most of the tactics remain the same; however, you do not need to lead your shots as much as the M6D due to the fact that the BR55 relies on a hitscan mechanic.
There is a playstyle known as the "four-burst tactic": three to the body and one to the head, As long as the opponent doesn't have an Overshield, and/or any other in-game health bonuses.
The BR55 returns in Halo 5: Guardians as a mythic loadout weapon. It is functionally very similar to its counterpart in Halo 2, and has a noticeably faster weapon-switch speed than other weapons. This can be used to more effectively replicate button combos from its game of origin.
The Halo 5: Guardians BR55 has a marginally slower kill time than its counterpart the BR85, but has much higher bullet magnetism and a faster reload speed. It also can enter Smart-scope faster.