For all things NFA -transfers, sales and such- I HIGHLY recommend "B3 Armament". Call (703) 200-0470. They are located in Chantilly. They have handled two completed NFA transfers for me and I am in the middle of the third. I cannot say enough good things about them. Very professional and very discrete. Once they receive your gun, you will get a chance to fondle it when you stop by to complete the paperwork then it stays in a climate controlled vault and NOBODY else touches it unless it is to verify a serial number or other official action. No bubbas hanging around the shop itching to finger-fuck somebody else's $10,000 machinegun. No parts getting swapped.
They are also always on top of their end of the paperwork and they will keep you informed and notify you of any ATF issues/actions as soon as they receive notice.
I don't work for them or anything. I have just been extremely pleased with their outstanding service.