General Discussion

If he bought them legally, it means he went to the local police department, where they did a background check and issues a permit to purchase.

He then went to a gun store, where he had to show said permit, and they did another federal background check before the sale. Or, he found someone with a used gun, they both went to the gun store, the store ran the background check for them, and the sale was made.

And if all this was done, the gun store, distributor and manufacturer would all be legally shielded from liability.

There's not much in this chain of command to prosecute.