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 your lone Grot" -WH40k nub
"Dude, my S10 AP2 Demolisher Cannon just totally pwunzorised your entire 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.
"Curses! My bolter's jammed!" -The ever-classic Warhammer 40k game Space Hulk