The Battle Rifle 55 Heavy Barrel is a gas assisted delayed roller blow-back rifle, a more advanced cousin to the M392 Designated Marksman Rifle. Using the much heavier and bulkier 9.5mm rounds, the BR55HB's intermediate range can't match that of the 7.62x51mm, but trades range for devastating terminal power, especially when used in conjunction with it's burst fire capabilities.
The BR55 Is an all time favorite rifle for me. When Halo 2 came out, I was very skeptical of the weapon and wondered why they abandoned the MA5B. But having played through the series, I decided that I really like the weapon.