[muir_redwoods]

So, Bill builds an AR variant in his workshop. It has no S/N nor brand name. Bill sells it to Tom. Who and, how enforces this rule and when?

[Scrambler Bob]

So, Bill builds an AR variant in his workshop. It has no S/N nor brand name. Bill sells it to Tom.
= = =

This rule does not change the current law, that Bill already cannot legally sell that gun.