BR55 battle rifle

Advantages
Designated the BR55 Battle Rifle, it has reasonable power, decent rate of fire, and high accuracy; proving itself to be one of the most versatile weapons in the UNSC arsenal. With a 2x magnification scope, it can be used to deliver three-shot-bursts accurately from medium-to-long range. Firing at any unshielded target's head (besides Hunters) results in an instant kill (see head shot). In Campaign, the Battle Rifle is useful against just about any infantry units at medium range. In multiplayer, three bursts bring down standard shields, while one headshot (meaning that only one of the three rounds fired needs to connect) results in death if the shields are down.

Disadvantages
At longer ranges, it is necessary to minimize strafing movements when firing to avoid "splintering" the shot grouping too much. Since each "shot" is really a three-round-burst, certain movements can break up your shot's grouping and greatly reduce accuracy. 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).

Influence
The BR55 is largely based on the US Army's XM8 assault rifle prototypes, and also has features resembling France's standard issue  FAMAS rifles.

Appearances
The Battle Rifle was not featured in Halo 1 but was created for Halo 2. It was designed to replace the Pistol from the original game; this was because the developers felt that the pistol was overbalanced for a weapon of such a small size (and just in general, as compared to the other weapons in the game).

The "BR" was originally designed by Bungie to fire single shots in semi-automatic rates of fire, but the idea was apparently used instead for the Covenant Carbine.

Another idea the developers were working on was a three bullet minimum per burst, and holding the trigger resulted in full auto. Zoomed in, the rifle only shot a single bullet. However, these features were ultimately scrapped in favor of the more simplistic design that was used in the final version of Halo 2.

The Battle Rifle will be returning in Halo 3. The screenshot below shows that the "new" Battle Rifle has a longer barrel, and it is widely speculated that this is a silencer. the optical scope appears different, and there is now a trigger guard over the trigger, a feature lacking in the Halo 2 Battle Rifle

Ammo
It is interesting to note the ammo used for the Battle Rifle. It uses a new 9.5mm x 40 Experimental Round. The 7.62mm x 51 round is a moderately large round used on the MA5B and on modern day marksman rifles and machine guns. The 7.62mm already has a history of having too much recoil because of the bullet's size which made it hard to fire fully-automatically in assault rifles. Having a bigger bullet that is almost 2 mm longer in width would make recoil worse. Due to this thoery, it would make sense to say that future technology might have some role in countering these effects. It could be safe to say also that due to the Battle Rifle's bullpup design, it could counter the recoil effects.

The size of the ammo would also have a problem fitting in a 36-round magazine of the current Battle Rifle. If an Heckler Koch G3 (which is chambered in 7.62mm x 51) has a standard magazine count of 20 and be the size that it is, it would be hard to see a bullet much thicker than the 7.62mm fit inside of a magazine of the Battle Rifle's size and with a capacity of 36 rounds. Again, this might have something to do with future UNSC weapon technology. This is a continous pattern with UNSC assault rifles as a similar instance occured with the MA5B. The MA5B has a magazine of 60 rounds chambered in the 7.62mm x 51 with a rather "small" magazine size.

Glitches
The Battle Rifle may be used in several glitches. Although they can be used with all weapons, the BXR weapons glitch allows a user to melee, cancel the animation, and then fire by tapping B (melee), X right afterwards, and then R (the right trigger/gun). This causes an instant kill to a shielded opponent (Elite in campaign or opponent in matchmaking).

Another glitch is the RRX, also known as the double-shot. When you press R (the right trigger)twice without pausing followed by pressing the X button right after the pressing of the second R, instead of firing a three-round burst followed by a pause, it fires six bullets without pausing, simulating automatic fire. After successfully pulling off the glitch, your weapon reloads but without any reloading animation. The drawback with the glitch originally was that once you did the double shot, you were reloading the weapon and therefore could not interrupt it and start firing the weapon again to perform another double shot. However this was solved by tapping the Y button twice to switch your weapon and to quickly switch back to your battle rifle, which interrupted the reloading procedure and you could thus start double-shotting again. It takes four bursts from the battle rifle to the head to kill a target, so successfully tapping RRXYYRRX while accurately firing gave you the ability to kill an opponent quickly, but it is very difficult to master.

There is also the RRYB, an instant kill glitch where you press the right trigger twice, immediately followed by pressing Y (which switched to your other weapon) followed by pressing B (melee) all while running towards your target. It is essentially a BXR but in reverse and you fire two shots instead of one.

Constant fire - This glitch is not easy to perform, but not hard either. The player must not have host connection of the party else it doesn't work. First, fire one bullet of the BR, next throw a grenade straight aay, then tap reload twice or three times quickly before resuming to fire, constantly. If it works you should be able to fire all your ammo without reloading. (RLXX(X)RRRR...)