The ATF told me...
I asked much the same at a seminar with sevaral agents and one of the attorneys present and it boiled down to this: If you assemble from already numbered and tranfered parts, then sell that firearm, it's considered manufacturing. Example: Buy an action and tranfer to your shop. Build a custom and sell and transfer to customer.... manufacturing. If you take a firearm and say for example change the grips and the sights then sell / transfer... manufacturing. So I asked, If I sell a action to customer make the transfer, and he or she then has me build a custom for them is that ok? The answer was yes. I think I have seen before some smiths advertise that way, that is "Custom work on your action"
I'm sure someone else has gone thru this before and would be interested what they were told. I am just getting started in the business and want to do things the right way, but have heard many different things from several FFLs.
Dale