The Bolter is a large, .998 calibre assault rifle. It has a much greater mass than most standard-issue rifles such as the Lasgun, although it is slightly shorter in length. Unlike most rifles, it lacks a stock, resulting in a grip much like a pistol's or a submachine gun's. A Space Marine's bulky Power Armour would make shouldering a stock an awkward experience, but Power Armour is most likely also able to compensate for vibrations and oscillation, making a stock redundant. Stocks are sometimes used, however, with an extended barrel and an M40 Targeter System to transform the Bolter into a sniping weapon system. These sniper Bolters, known as Stalker Pattern Bolters, are most often used with Stalker Silenced Shells.
Bolter ammunition (a bolt) is primarily a solid slug. Conventional solid slugs utilise a propellant charge contained in a casing that, when ignited, forces the bullet out of the barrel. In contrast, a bolt is self-propelled; it features its own integrated solid propellant that propels the bolt at high speeds, essentially acting like a miniature rocket. The propellant itself is shaped to control the bolt's direction and speed; however, this method of rocket propulsion would normally warp the barrel due to gas pressure. The Bolter uses an ingenious method to prevent this.
As well as the rocket propellant, a tiny amount of conventional charge is also utilized. This charge is just strong enough to force the bolt out of the barrel and ignite the bolt's propellant. The rocket-propellant is carefully fused to ignite just after leaving the barrel, alleviating any possibility of pressure build-up. The bolt then accelerates away towards the target under its own power.
The standard Bolter ammunition is designed to penetrate the target and then detonate, causing immense damage to the target and leaving little opportunity for survival.