9mm AR's are straight blowback. They have NO gas system, so "piston or DI gas system" is irrelevant.
I would just build it as a pistol and add the stock when you get your form 1 back.
You need to engrave your trust name, city and state. Has to be readable and .003" deep. No particular place on the receiver, but has to be visible without disassembly.
Be aware that some folks have had OOB discharges with the 9mm AR's and Slidefire stock. One member on here mentioned that he had this happen, but I do not remember who it was.
I believe that since your lower already has manufacturer makings you would fill this out in the boxes "4a", "4g", and "4h". No name or address engraving would be needed. That is, unless you want someone to track down where you and your guns are. If you were making a gun from scratch and no makings were on the gun then you would have to mark it. Correct me if wrong.
No sir, that is a common misinterpretation. You do NOT have to mark an already existing weapon. That only applies to making a new receiver from scratch or making a can etc. I have a close friend who has been dealing with NFA weapons for years and informed me on this when i was doing my uzi. I even called the ATF and they said since my receiver was an existing gun with the manufacturers info already on it i did not need to engrave my info. I filled out the info and serial on the gun and that was it. Got the approved papers and no ugly scribbles on my gun. Read this from the online ATF publications of the national firearms act handbook chapter 6:
The law you quote can be interpreted in different ways and most likely is directed at licensed manufacturers. If the NFA handbook isn't accurate then i don't know what is. Its either someones interpretation of the confusing law or the clear explanation given by the ATF themselves in their handbook. I know what i'll beleive. I encourage everyone to study the NFA handbook. And remember, if you are a non-licensee then chapter 6 applies to you.
http://www.atf.gov/publications/firearms/nfa-handbook/
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