The apotheosis of bullet weaponry, the boltgun is a perfectly-crafted firearm gifted only to the Emperor's finest. Boltguns fire shells which explode shortly after penetration, inflicting massive tissue damage.
"Dude, my massed S4 AP5 boltguns totally pwunzorised yourlone Grot" -WH40k nub
"Dude, my S10 AP2 Demolisher Cannon just totally pwunzorised yourentire squad of boltgun-nubs" -Slightly more fortunate WH40k nub
The more common term for boltgun.
Note that heavy assault troops tend to make use of a double-barelled version known as a storm bolter, and support troops and vehicles extensively utilise heavy rapid-fire versions known as heavy bolters.