re: How has the gun not been more traceable. It is not easy to find untraceable guns for most

The gun used was intentional, if in fact it is the gun that was used.
It is old and probably never went to an FFL and even if it did at some point during transfer, it could've been sold through private sales with no paperwork if done in a state that allows that.

Now if this gun was sold somewhat recently to this guy, maybe the seller would recognize him and have some correspondence with that person. I suspect if you are gun owner that knows how this process works, you would take actions to avoid that gun getting traced to you.

I really don't understand why using that gun was what he decided. An AR-10 could've easily been carried in the backpack and disposed of easily in a dumpster somewhere. Why leave it in the woods to be found, why risk having to bring in a long rifle in your pants which is very suspicious.