Automatic and semi-automatic weapons work the way the do because force of the round (recoil) pushes back the bolt carrier, which a spring will then push forward again. Shot is fired, bolt carrier goes back, spring pushes it forward.
Subsonic ammunition have less charge than regular ammunition, to reduce the velocity. This also means less recoil. Combined with the spring now being too stiff, the bolt carrier will simply not move back far enough to successfully chamber a new round. So you have to manually chamber a new round between each shot. One solution is to use a light / less stiff spring that is adjusted to the force of the subsonic ammo.
Perhaps subsonic rounds that did not have enough charge to drive the slide. It looks like there may be a suppression device on the end. My first thought was home made.
Just an interesting note. Some in the media have suggested the weapon used is a B&T Station Six, a bolt action pistol. I had no idea such a thing existed. Reviewing the footage again I don't know if it is that model. The hand movement appears more chambering that cycling a bolt.
Or it could just be a single action pistol, as noted in the article. They’re not common but there are a handful of them that are relatively easy to obtain. Competitive shooters use them and it seems that Seals are sometimes issued single action pistols too.
Or someone highly motivated to make the hit a work of art. Killers are usually not very smart, or they are blinded by rage, and they make mistakes and are sloppy and incomplete about planning.
