'Positron rifle' is a term describing any long positron weapon made for engagements over a great distance. Positronic weapons fire 'bolts,' which are slugs of solid positrons enveloped in an energy casing. When the bolt strikes its target, the energy casing burns off into the target so the positrons can be exposed. When these unsheathed positrons come into contact with the electrons in the target they annihilate each other, causing severe, localized disintegration.
The Heckler and Wesson PR-12 is a battle proven assault rifle capable of delivering dense positron bolts at great ranges. Unlike modern positron rifles, the PR-12 doesn't rely on an internal energy source to launch its projectiles. Instead, it uses a hand crank on the right side of the weapon which also happens to play "Pop! Goes the Weasel". The PR-12 has been in service since 2972, and the Earth Army is currently conducting field tests for its replacement.