The Westinghouse M-27 Phased Plasma Pulse Rifle is a plasma-based weapon used by Skynet forces in the Future War. It has a bullpup configuration and is in the 40 watt range. Introduced in 2015, it replaced the F-2000 as Skynet's main sidearm.
It is possible that the under slung barrel is a form of laser used to ionize a channel through the atmosphere to the target making transmission of the plasma bolt easier.
T-800 with Westinghouse M-27 Phased Plasma Pulse Rifle