What kind of gun is the silver one he's holding? That thing looks like it would pack a punch.
Off hand I'd guess a .45 caliber, or a .357 or .44 magnum.
If it is a .357 or .44 yeah it would cause major damage and even if they were wearing a vest that could stop the round it would still knock the average person on their ass. Not that a .45 caliber won't do damage but the other two calibers typically have noticeably more muzzle velocity and kinetic energy.
I'd say they either stole it or bought it from someone who stole it.
These guys are just as much thugs as Mr. Dunn is. A person who isn't a thug won't neglect to call the authorities when they get involved in a shooting. Mr. Dunn didn't call the police and I don't think these guys did either.