I just did one of these recently, using Pete's Gun & Tackle in Hudson,NH. The seller's part of the transaction was simple and fast; any FFL in NH who is anywhere near the border with MA should be familiar with the procedure.
One of the things the owner of the gunshop told me (don't know if it is true, was just making conversation while waiting for NICS), is that if the seller gives me a copy of the FA-10 he got when he originally bought the gun, that having a copy of the old FA-10 would make it easier for me to transfer that gun to a MA resident (via an FFL in MA) later on?