Give Bob Bigando at Hamilton and Sons firearms (Cottonwood AZ) a call. They're currently putting a Vulcan barrel on mine, when done she'll shoot 20mm vulcan with the cartridge rim turned down.
I just had one approved for a c&r DD took about 9 months. Also had one dissapproved back in January which was at 14 months, same deal c&r DD. So from my limited experience seems to be on par with non c&r form 4's.
Just received a rejection on a form 4 due to the serial number being wrong. When the gun was transferred to the owner in 1984 they conditionally approved it under the condition that the serial number was the year it was essentially the year it was manufactured (it's a DD BTW). Now, fifteen months later, they bounced it and are digging their heels in over the serial.
USPS will accept from non FFLs as long as it's a long gun. It's in the regs, although had one post office tell me no. Went to another and after about twenty minutes they accepted it. Post master called after I left asking me to come back and repackage it more discreetly (it was in a rectangle box). Went overnight to the other side of the country at half the price of what the UPS and FedEx quotes were.