Here is a guide to properly SBR your lower. I put this together due to the rash of “mishaps” that seem to plague people who are SBRing their lower.
1. Live in a Free State
2. Realize that you will be married to this lower probably for the rest of your life and you will get pennies on the dollar if you try to sell it after the SBR process.
3. Buy a quality lower preferably from a company that will service it after it has been SBRed. Olympic Arms doesn't count and neither does any cast 1980s crap.
4. Check all the pin holes to see if they are in spec. Trigger/hammer holes are .154” and takedown holes are .248”. The tolerances are +/- 0.001”. Anything more than that is out of spec. I don’t care what any blueprints say. You can take 100 lowers and measure them yourself. You will come up with these numbers.
5. Measure the magazine well. A good magazine well will measure .900” at the front and back and .970” at the slot. If you are less than this, you may have magazine drop free problems.
6. Assemble the lower with a quality LPK such as Colt or LMT. Use a quality stock with proper buffer.
7. Find an good carbine upper of known functioning. Better yet, two or three good carbine uppers that function well.
8. Place carbine upper and lower together and fire a minimum of 500 rounds through this combo to make sure you lower is good. Preferably use a variety of magazines.
9. Disassemble lower.
10. Find a good engraver. Orion Arms and Indent are good. Your local trophy shop/jewelry store is not so don’t waste your time, neither is your Dremel tool.
11. Send lower off for engraving. You do this before you send in any paperwork to the ATF in case something happens to your lower during shipping to the engraver or damage at the engraver. You don’t want to explain to the ATF that your NFA firearm is lost in the mail. Anybody that does this backwards is a complete idiot.
12. Get lower back and make sure everything is ok.
13. Send in NFA paperwork
14. Get approved NFA paperwork.
15. Get less than 16” barrel/upper.
16. Assemble your gun and enjoy.
Anybody that follows these steps will not have any problems.