The TR-116 is a firearm originally designed by Starfleet for use in environments where phasers do not function properly. It appears only in the DS9 episode "Field of Fire," making it a technology of the week.
The TR-116 is similar to modern firearms, using a chemical explosive charge to propel a metal projectile toward its target. According to Chief O'Brien, the weapon was originally designed "to be used in energy dampening fields or radiogenic environments where ordinary phasers are useless," which once again highlights the limitations of phaser-based weaponry. Starfleet reportedly abandoned the weapon when "regenerative phasers" were developed that were less sensitive to adverse environments.
The TR-116 in the DS9 episode, however, included additional features that O'Brien claimed were not part of the original design. This weapon included a sensor system that allowed the user to see targets that were not in his or her direct line of sight. It also incorporated a miniature transporter system that could dematerialize the bullet as it left the weapon's barrel and rematerialize it right next to its target, allowing an assassin to literally shoot through walls.
With its sensor system and microtransporter, the TR-116 appears to be an ideal assassination weapon or long-range sniper rifle. O'Brien claimed that the microtransporter was not part of the original design, but he demonstrated the microtransporter feature to Odo and Ezri Dax before seeing the actual murder weapon used in the episode, indicating that he was already aware of the existence of the additional option. The existence of the device apparently was not common knowledge even to Starfleet personnel, indicating that it was a Federation secret, possibly intended for use in covert operations such as assassination or anti-terrorism operations (a bullet beamed through a wall could potentially kill hostage takers with relatively little risk to the hostages, even if they used human shields).
Furthermore, the weapon apparently used large-calibre bullets fired at subsonic velocity, and it had a small ammunition clip. These characteristics further indicate that it was designed for stealthy, long-range sniping rather than conventional combat.
- Mike Wong's discussion of Federation small arms, including the TR-116.