Thats a Real fine line.
Legally you need to have a 07 firearms manufacturing license and also a 02 SOT Just to make parts for suppressors. but these are solvent traps persay. however people know what solvent traps are used for.
where it gets iffy is when a guy shops out the part above to a machine shop that has no clue what it is as to them there just parts.
if the feds wanted to push it they would win. I think the only way you could get off on it is with a good lawyer and not having any clue about weapons.
I have a clue about weapons and supressors but didnt put 2 and 2 together until Years ago when the Solvent traps came out, we had a guy ask use to make 1000's of parts. pricing was kinda cheap but was tempting. told him I would do the small order and see how it went and ordered material. about 2 days later he goes I need you to quote this tube also. I looked at it and was like is this for a supressor, he goes NO its a solvent Trap. quick search on the net then I knew what they were. I ended up calling the local BATF to find out if I could legaly make them. Guy said technically yes and no but if the customer was to get into trouble they would be knocking on my door as well.
he never asked who the customer was.
I canceled the order cause I didnt want to risk it. I just didnt need any crap down the line.
I know when I was asking about getting my 02 SOT a few years back talking with the ATF the said DONT make ANY PARTS for one untill you get your license. sames goes for the form 4(do it yourself suppressor) you dont make any parts till you have the form.
I'd love to see ATF touch/rule on this(and maybe they have in the last 4-5 years) cause innocent machine shops can get into some nasty crap by not knowing.
Ive made a ton of gun parts, all kinds its legal with out a manufacturing license , whats not legal is receivers(the gun part ie serial number part) if your talking ar style that would be lower is not legal but upper is perfectly legal with out a license. unless your building them for yourself.