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.
https://abcnews.go.com/US/police-piece-unitedhealthcare-ceo-...