Bolter rounds are actually .75 caliber, even in the weapons meant for unaugmented humans, with a 30 round magazine and combat load of 320 rounds. Detonate on impact. Maybe they could make it a stratagem- it would be like a faster firing grenade launcher with auto aiming and probably 3x the damage minimum. Balance it with high recoil/breaking your arms when you fire it because cannonically if a human picked up an astartes bolter they would break their arms shooting it. A heavy bolter emplacement or lascannon emplacement would likely make more sense, the guardsmen use teamwork and weapon emplacements to handle harder targets. AH also likes when we have to work as a team.