Welrod

The gun used in the United Healthcare CEO killing was not a Welrod but instead a poorly set up semi-automatic pistol.

To get suppressors to approach the kind of quiet operation you see in the movies in real life, the shooter needs to use subsonic ammunition. That's because suppression of the firing noise isn't beneficial if the bullet immediately breaks the speed of sound, making a loud crack.

9MM rounds (the caliber used in the United Healthcare CEO Killing) are supersonic by default. Subsonic ammunition can be purchased or made for 9MM guns, but the way to make the round subsonic is to use less gunpowder on a heavier round.

A downside of this is that using these rounds in a standard unmodified semi-automatic pistol will often result in the gun not cycling. That is to say, the kinetic energy of these lower gunpowder bullets leaving the weapon is not enough to cause the gun to eject the spent round and load the next round.

So, the round will fire, but to eject the spent casing and load the next round, the shooter needs to manually cycle the firearm, as seen in the surveillance video in the recent incident.

Welrods are bolt action, they intentionally don't attempt to cycle automatically after each firing. The weapon in the video shows gas escaping the chamber, something a bolt action weapon would not do.

Someone with serious firearms training and expertise would have modified the weapon to cycle with subsonic ammo or used a .45 caliber weapon that is subsonic by default.

The Welrod has been getting a bunch of press in the last couple days because of speculation that it was used in the United Healthcare shooting. Noted Gun Expert Ian McCollum of Forgotten Weapons has opined that it is definitely not[1], but it's an interesting piece of history nonetheless, and a ton of people have probably heard the term without having any idea of what it means.

[1] https://youtube.com/shorts/POubd0SoCQ8