You've been watching too much TV. "Ballistic matching" is a myth. And moreover: You think there will be .223 bullets found in bodies? It's a 3400 ft per second round, depending on load. It goes right through. I've seen it go right through a deer, and human body is similar.
There will be many bullets found. The place was a ricochet heaven with concrete walls. And ricochet bullets can stay in bodies as they are slowed down. Bullets will be embedded in concrete. And the other guns might have different calibers. Also bullets that have already gone through a body or two may stick in the third body for example.
I’m not talking about ballistic matching necessarily.