If you do solder it must be at least 1100 degree. If you pin it, you must weld a tack over it. I think there is a requirement for when you are just welding the base of the muzzle device to the barrel.
I'm not sure how having the right tools makes it possible to do it legal. I'm obviously missing something when I read that statement.
You can trust a bunch of guys on interweb, or you can go talk to / do business with a Class 2 FFL. I mean what's the worst that can happen if you get caught with unregistered SBR? ( hint: 10 years in federal slammer, $250k in fines, plus whatever state charges you get ....)