http://www.atf.gov/publications/downloa ... pter-2.pdf
Per the BATFE "...Permanent methods of attachment include full-fusion gas or electric steel-seam welding, high-temperature (1100°F) silver soldering, or blind pinning with the pin head welded over..." Tack welding is not permanent enough in the ATF's opinion.
If the lower is separated from the upper (or the barrel is separated from the serial numbered part, then it's not a SBR until you are in possession of both. The safest ways are to do the paperwork before doing any cutting or welding, or, to have the barrel someplace where the receiver is NOT. I always used my brother's house as my 'out of my possession' spot.
Per the BATFE "...Permanent methods of attachment include full-fusion gas or electric steel-seam welding, high-temperature (1100°F) silver soldering, or blind pinning with the pin head welded over..." Tack welding is not permanent enough in the ATF's opinion.
If the lower is separated from the upper (or the barrel is separated from the serial numbered part, then it's not a SBR until you are in possession of both. The safest ways are to do the paperwork before doing any cutting or welding, or, to have the barrel someplace where the receiver is NOT. I always used my brother's house as my 'out of my possession' spot.