Described as “nothing special” by FBI, a DPMS AR-15 5.56 probably $400 or so when purchased. Roof video after Crooks neutralized looks like it has some kind of small Aimpoint-style sight.
DPMS has long been a supplier of kits, uppers, lowers, makings of all sorts, including out-of-spec factory barrels. I bought a few of their trigger group parts decades ago and found them to always be out of spec. They bought from the mil-spec makers, taking their out-of-grade stuff. Serviceable but never exactly right. Knowing the rifle was/is DPMS would lead me to expect it couldn’t actually make a reliable head shot at 130 yds. And that could explain the two wounded guys along with Corey who seem to be victims of scatter-shot?
It’s not a small job to make a consistently accurate 22 cal barrel that can handle mach 3 or more.