As I understand it, lower receivers are bought on the 4473 as not as a "long gun" but as an "Other" category subject to the 21-year age limit same as handgun or a PGO shotgun.
Converting a Title I long gun to a handgun is a no-no, because it becomes a Title II SBR or SBS, requiring an approved Form 1. The intent of Congress being to prevent evasion of handgun regulation by sawing off less regulated rifles or shotguns to make concealable weapons.
However, it has been OK to convert a handgun to a long gun by replacing the barrel with a 16" rifle or 18" shotgun barrel and adding a shoulder stock.
Since a firearm legally bought and owned as a handgun is not an attempt to evade handgun regulations, converting it from rifle configuration back to a handgun should not be counted as a violation. Letter 2011-4 signed 25 Jun by ATF Acting Director Kenneth E. Melton 25 Jul 2011 appears to support that.
What would be violations (without a Form I) are:
A. pistol version with shoulder stock and short barrel attached, or with shoulder stock available w/o long barrel or
B. rifle with short barrel attached, or with short barrel available w/o pistol grip.
What is interesting is the idea that since a lower is sold as "Other" firearm, the lower could be configured as a rifle or pistol, and switched, as long as a Title I rifle or pistol is created, and not a Title II stocked pistol or short barrel rifle. I want to see how this plays out over time.
