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.
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