The BR55HB SR

The BR55HB SR is a select-fire rifle that has been in service for over 27 years. The UNSC employs the Battle Rifle as a medium range marksman rifle, it fires new 9.5x40mm ammunition from a 36 round magazine, which fits flush in the receiver. The magazine housing is built directly into the underside of the stock of the rifle and is located behind the grip. This titanium alloy weapon is gas-operated with a rotating bolt and therefore must be charged before the first round can be fired. The charging handle used to chamber the round is located on the left side of the weapon and does not move during operation, but is pushed back during reloading. The rifle has a housed magazine in the stock. The ejection port is located on the right side of the weapon. The BR55HB SR, has noticeable changes from the BR55. It has a longer barrel for increased range and accuracy. It is also fitted with a Trijicon ACOG-style scope on a modified carrying handle and now also has a trigger guard. The safety is also located above the handle of the weapon.
It is never stated exactly what the maximum effective range of the BR55HB SR is. The MA5C Assault Rifle, which was described as most suited for short to medium range, as opposed to the Battle Rifle's mid to long, has a maximum effective range of 300 meters and the Covenant Carbine, which is similar in use with the Battle Rifle, has a maximum range of 600 meters. It is stated that the Battle rifle could reach up to 900 meters with deadly accuracy; however, that may have been due to the design of the prototype Battle Rifle, deployed in 2524, the field issued Battle Rifle may not have that type of range.I have got this info from halopedia.com