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Yes, any barrel length under 16" is considered a pistol/sbr depending on Tax stamp status
Not always a pistol or SBR. You can also build an “Other firearm” it’s not a pistol because it maintains a vertical grip and is designed to be fired with 2 hands not a rifle because it does not have a stock, it maintains a pistol brace. And the firearm must have an overall length above 26 inches in its smallest configuration. Also has to be built from a stripped lower that has never been a rifle or a pistol. This gets around fixed magazine laws in a lot of states with unconstitutional laws.
Yes. the only way to make this a non-sbr would be to pin and weld a muzzle device or a suppressor. I don't know of any flash hiders that are going to give you the 3.5 inches you need. Surefire Socom suppressors are right around 6 inch. quite honestly if you went that route you might as well just register it as an sbr. need a tax stamp either way. an easier way would be to get a 14.5 and pin and weld a surefire warcomp. that gives you your 16in + and you could throw on a suppressor after the fact.