Many viewers of the surveillance footage of the murder of Brian Thompson perceived the assassin to be using a semi-automatic pistol and to have experienced multiple cartridge jams that required him to cycle the bolt manually. More likely he used a non-auto-ejecting, single-shot bolt action pistol. The advantage of this design is that it drives all of the combustion down the barrel and into the integrated silencer instead of bleeding gas off to cycle the bolt, which is considerably louder.

In other words, the assassin apparently favored silence over speed, and the weapon he chose may have been the following (see graphic above)

As the weapon is described on the manufacturer's website.

This updated, integrally suppressed pistol features a new grip and updated magazines, yet maintains its non-descript appearance, whisper-quiet sound signature and unique rotating bolt operation. This model in 9mm wears a 3-in. barrel. It finds its operational success as a magazine-fed, non-auto-ejecting single-shot functioning in a rotational bolt action.