Here is what I did for my SBR + Trust:
1. Have attorney draft a gun trust
2. Buy stripped lower
3. Buy 10.5" upper
4. Mail lower to someone that knows how to engrave per ATF regulations
5. Wait for the lower to be returned and verify everything looks okay. The name of your trust, city and state should be engraved on the lower
6. Build-out the lower as a PISTOL. Per ATF you can use a standard buffer tube, just take the stock off before attaching the tube to the receiver
7. Attach the upper and test fire your PISTOL to ensure you are happy with it
8. Complete ATF form 5320.1
8a. Check box 1A
8b. Check box (2) Corp or other business entity
8c. In 3b Enter the name and address of your trust
8d. Complete 3d and 3e
8e. In 4a put the original AR lower manufacturer information
8f. In 4b put "Short Barrel Rifle (SBR)"
8g. In 4c put "5.56MM" if that is your caliber etc...
8h. In 4d put the original mfg model as engraved on the lower
8i. In 4e put 10.5" if that is what you bought
8j. In 4f put 31" (if your barrel length was 10.5")
8k. In 4g put the original mfg serial number
8l. In 4h list the other markings on your receiver. I also added: (4.c continued)/.223/9MM/6.5MM/6.8MM/(caliber "Multi")
8m. In 4i enter "Any lawful purpose"
8n. In 4j check "No"
8o. Leave 5 & 6 blank
8p. In 7, sign your name
8q. In 8, print your full name followed by ", Trustee"
8r. In 9 enter date of signature
8s. Answer "No" to everything under question 10 & 11 then stop
9. Print 2 copies of the form. Print page 1 then flip the paper over and print page 2 on the back - Do this twice as you need to mail 2 copies (2 pages printed front to back)
10. Complete ATF form 5330.20. Some folks say you don't need this when doing a trust. I did it anyway. If they don't need it they will trash it. Read the instructions. Print in all upper case and spell everything out. No abbreviations.
11. Create a "Schedule A" (or whatever your attorney called it). This is a list of inventory owned by the Trust. First, fund the trust by transferring $1.00 cash into the trust. Next, add your soon to be SBR to the Schedule A. In the comments column (which I added), I printed "Pending BATFE Approval"
12. Go to the post office and get a money order for $200.00. Make it out to the "Department of Justice". The Money Order should be from your Trust, not you
13. Make a copy of your full Trust and Schedule A
14. Mail the Certificate of Compliance, the 2 copies of form 1, your Trust and the money order to the NFA branch (address on form 1)
15. In ~7 months you'll get one copy of your form 1 returned with a cancelled stamp on it. This is your approval.
16. Reattach your stock to the buffer tube. Rock on!
1.Build as pistol so you can use the gun while you wait 7 months.
2.Don't send the paperwork to the ATF until you get the receiver back from the engraver and you have assembled + test fired to ensure everything works, otherwise you have to go back to the ATF should, for example, your lower get lost in the mail or you break the lower while hammering pins in-place.
3. Some folks talk about "constructive possession" of a SBR by having a AR pistol in the home with a stock laying around. This doesn't concern me but if it does you, leave the stock with a friend until you get your stamp.
4. If you don't go the Trust route and register in your own name, you will need: Fingerprints, Chief LEO sign-off (both forms) and passport photos (both forms). The feds will do a background check which might further delay getting approved.