The SRS99 AM is a gas-operated semi-automatic UNSC sniper rifle that fires 14.5 x 114mm APFSDS (armor piercing, fin-stabilized, discarding sabot) from a 4-round magazine. It is designed in the conventional layout, so the magazine and operating system are located in front of the trigger. The magazine does not fit flush.

The SRS99D is about 187.5 cm long and is completely interchangeable. From the barrel to the breech to the stock, everything can be changed to fit the current situation and satisfy the operator. The SRS99D in Halo 3 has some notable changes from the SRS99C models. First off, it is fitted with a 5x and 10x optic that shows real time images in infrared vision when not looking through it. When looking through it, the scope shows a target's distance and elevation.

The SRS99D also has an altered foregrip, stock, body and sports a trigger guard. The barrel also now has an iron sight. This weapon does not possess a safety and has a frame that serves as a carrying handle as well as a scope protector and a folding bipod located in front of the magazine on the underside of the rifle.