BR55 battle rifle
From Halopedia, the Halo wiki
The BR55 Service Rifle, also known as the Monkey Wrench, BR, and the Big Boy, is a UNSC Armed Forces infantry battle rifle. It is notable for its powerful three-round burst fire and accurate 2× scope, making it more efficient at farther ranges than the MA5 series assault rifles.
The BR55 Service Rifle is a gas-operated, magazine-fed, mid-to-long range weapon capable of semi-automatic, fully automatic, and burst-firing modes. The BR55, having a rifled barrel, is 89.9 cm long and is fitted with a scope for increased accuracy. The scope is mounted on the optics rail. The safety is also located above the handle of the weapon. The weapon fires the M634 X-HP-SAP round from a 36-round magazine. Additionally, the weapon features a scope attachment capable of 2× magnification. In addition to its initial role, the BR55 battle rifle can also fulfill the role of a designated marksman rifle.
XBR55 Battle Rifle
BR55 Heavy Barrel
The weapon's heavy-barreled variant, the BR55 Heavy Barrel Service Rifle, entered service in 2548, superseding the M392 Designated Marksman Rifle, which had been steadily phased out in favor of the BR55, in service in all branches of the military with the exception of the Army.
The BR55 has reasonably high power, a decent rate of fire, and high accuracy with an electronic 2× telescopic sights attachment; it can be used to deliver three-round bursts at medium ranges. In multiplayer, three body shots and one head shot will kill a fully shielded opponent, assuming the player does not have an overshield. This weapon will kill an unshielded enemy in a single head shot. Target the head and if you're within range a red dot will appear in your reticule. This requires that only one of the three rounds from a burst actually hit and kill an enemy. Therefore, it is possible to kill up to three unshielded opponents in a single burst if they are grouped close enough to each other, though this is an uncommon occurrence. However, right after you shoot, it is possible to only shoot one round rather than three bullets, with a melee attack to disrupt the burst. Against vehicles like the Wuzum-pattern Spectre or Karo'etba-pattern Ghost, an experienced player can easily take out the driver, rendering the vehicle immobile, however, this is harder with the Warthog.
In Halo 5: Guardians, the BR55 provides a slightly faster weapon switch and reload time compared to the BR85N. Most notably, however, the BR55 has higher bullet magnetism, which allows targets to be hit with less-than-perfect aim.
At longer ranges, it is necessary to minimize strafing movements when firing to avoid "splintering" the shot grouping too much. Since each "shot" is actually a three-round burst, certain movements can break up your shot's grouping, and could greatly reduce accuracy. To avoid this, either swap it in for the Covenant Carbine, or get up to medium range. The Carbine has the same degree of firepower, and it lacks the splitting effect although it is slightly less accurate. This also affects accuracy when firing from a moving vehicle such as a Warthog (especially when firing at a 90-degree angle from the vehicle's heading).
The BR55 is not as effective at long ranges. Each bullet also acts as a tracer which gives away the shooter's position. Also, the range is limited compared to the Sniper Rifle. It is also not effective against Sentinels, and does negligible damage against vehicles. Neither is it an effective weapon at close range, as the low fire rate and average stopping power of the weapon makes any fully-automatic weapon far superior in close-quarters combat.
Compared to most other UNSC firearms, those using the BR55 have access to limited amount of ammo as they can only three spare magazines, not including the loaded magazine in the rifle.
Non-canon and dubious canon appearances
List of appearances