The weapon had the appearance of a double-barreled pistol. It fired energy bursts that had an immediate effect on its target.
According to The General, due its status as the sidearm of the Lord President's personal guard, it possessed no stun setting and would be fatal to anything shot with it, even Time Lords. A single shot to the chest of a Time Lord with the pistol would force an immediate regeneration. The gun was presumably fatal for any Time Lord that had reached the end of their regeneration cycle. (TV: Hell Bent)
During the Coup against Rassilon, the General was armed with one of these guns, though he kept it holstered for most of the coup. Finally, when the General sided with the Twelfth Doctor, he symbolically dropped the gun to the ground before moving to the Doctor's side.
After tricking the Time Lords into using an extraction chamber to remove Clara Oswald from her timeline moments before her death, the Doctor punched the General in the face and stole his gun as part of the Doctor's plan to save Clara. The General attempted to talk the Doctor down and refused to allow him to leave. The General pointed out that the gun was the sidearm of the Lord President's personal security and as such possessed no stun setting. Though the General obviously expected this revelation to stop the Doctor, it only gave the Doctor pause for a moment as the General and Clara attempted to talk sense into him.
Though Clara asked the Doctor to put the gun down, he asked the General which regeneration he was on. The General realised what the Doctor intended and told the Doctor that he was on his tenth regeneration and had thus not reached the end of his regeneration cycle. After the Doctor and the General wished each other luck, the Doctor shot the General in the chest, causing the General to collapse to the ground. By the time Gastron arrived in the extraction chamber moments after the Doctor's departure, the General had already begun a regeneration into a female incarnation because of the shot. The Doctor soon discarded the weapon at Clara's insistence. (TV: Hell Bent)