The early Imperium was built largely on the power of the boltgun.
While largely replaced by energy rifles in present day, various forms of boltguns still find their uses with the Imperial armed forces and the civilian populace.
A boltgun is essentially an automatic rocket-propelled grenade launcher, firing rocket-propelled high-explosive projectiles rather than purely ballistic slugs. The origins of boltguns can be traced back to 1960's "gyrojet" prototype weapons, which employed the concept of spin-stabilized rocket-propelled projectiles. These initial prototypes were a failure in multiple respects. While the end velocity of the projectiles did exceed that of conventional rounds, the muzzle velocity of gyrojet weapons was extremely low, literally making it possible to stop the outgoing round with bare hand, which effectively made the weapon useless in ranges under 20 meters. Furthermore, while the rocket propulsion did enable a flatter trajectory and reduce weapon weight by allowing gun barrels to be made thinner, the production cost of a single round vastly exceeded that of readily available and more reliable conventional munitions. The existing manufacturing technologies also prevented rocket-propelled rounds of such small size from being made reliable, and the technology was consequently discarded until mid-21st century.
The next step in firearm evolution that eventually led to the development of the boltgun were smart grenade launchers devised in early 21st century. Since these grenade launchers relied entirely on low-velocity ballistic propulsion, however, they cannot be considered early forms of boltguns, though their design significantly influenced the later concept, which used ballistically-boosted rocket-propelled medium-velocity ammunition.
The advent of reliable mass-produced powered armor led to renewed interest in gyrojet technology by the Terran Confederacy in the 2030's. Since powered armor enabled troops to carry considerably heavier weapons than regular troops, the Confederate military was keenly interested in maximizing an infantryman's firepower without significant increase in weight and size of weapons.
Gyrojet technology readily addressed the firepower issue, technically allowing much more powerful explosive projectiles to be fitted within infantry-caliber firearms. Confed engineers resolved the low muzzle velocity problem by settling for a compromise, combining conventional cartridge with a rocket-propelled round that ensured the muzzle velocity of the projectile would be sufficient for a lethal impact, the rocket motor accomplishing the rest. This innovation allowed for unprecedented range and accuracy in excess of 1000 meters even in conventional infantry arms. Furthermore, rocket-propelled projectiles were easier to guide than ballistic rounds, enabling various sorts of self-targeting "smart guns" to be produced. Because of the reduced barrel thickness, the weapon did not suffer from increased weight, enabling the explosive payload to be maximized.
This consequently led to the development of .998 Godwyn, also known as "the gun that unified Mankind" and "the gun that humbled the Skargh".
The .998 Godwyn-pattern boltgun was first produced in 2029 and quickly adopted as the standard issue for Confederate troops in the wake of WWIII. The devastating firepower in combination with other innovative technologies enabled Confederate forces to crush the United States, and later China and India in WWIV.
With the onset of the Great Pilgrimage, the ancestors of Sidhae brought boltguns to the worlds they settled. The future Federation of Mankind, having lost the advanced powered armor technology and consequently resorting to lighter combat exoskeletons, settled for a different branch of firearms development, the pulse rifle.
The .998 Godwyn made it's comeback in the Skargh Wars, again inflicting devastating casualties upon the Skargh forces that were unaccustomed to fighting technologically advanced opponents on even terms.
More refined patterns emerged later on in the Age of Peace, ultimately leading to the legendary Kraken and Fenrir patterns that formed the mainstay of infantry arms during the Age of War, energy rifles being a novel and then fairly uncommon introduction.
Boltguns were gradually, but not completely, phased out of use following the Age of War, mainly because of the large amounts of material it took to supply them with ammunition. While boltguns have the distinct advantage of being able to fire various types of specialized munitions, their reliance on a steady ammunition supply, and their limited range and inferior accuracy compared to energy rifles led to their falling out of favour with the military. That being said, boltguns still remain in use by the Sidh civilian populace, and also in limited use by the military. The Imperial Army still widely uses heavy boltguns, the rocket-propelled equivalent to machine guns, as rapid-fire weapons for delivering suppressing fire, something the energy rifles are generally incapable of doing effectively.
In the post-Age of War era, boltguns are generally found in the use of colonial militia and Freelancer forces, seeing limited use with the military and security services asides from the aforementioned use as heavy weapons. They are liked for their ruggedness and simplicity, and the ease of manufacture compared to energy rifles.
Boltguns are generally designed for use with powered armor, making even the smallest typical models too bulky for comfortable use without one. They should not be confused with blasters, which are essentially conventional ballistic high-caliber arms firing explosive shells, as opposed to the boltgun principle of using a rocket-propelled projectile.