I am sure an ffl would be required if there is a transfer of ownership, but I thought it was legal to ship the firearm without intent to transfer, so long as it is made inert by shipping the gun without the bolt if its a rifle, without a cylinder if its a revolver or without the slide if its a pistol. and the remaining peice shipped seperately.
At least that is how most firearm companies ask you to ship your firearm to them for repairs. and how they send it back to you.
Springfield Armory, Smith & Wesson, Taurus, and Rock Island Armory never once asked me to do this for warranty or smithing work.