DOCTORPC said:
if your not sending to the manuf in other words just another gunsmith it has to go through an ffl
I do not belive this statement is correct. Been there, done that. I just boxed it up and shipped via UPS with insurance. Once the custom work was completed, the gunsmith simply shipped it back to me via UPS. Did this 3 times I can recall.
Just make sure you do not use a PO Box address either direction, and pay the extra for tracking and signature required so you do not find your package sitting outside at your door if nobody is home for the return shipment.
If you are really concerned, you could ship it in two different pieces, such as frame in one and slide or cylinder in the other.