Pistols can become rifles.
Rifles cannot become pistols.
Quirky, I know. Lemme 'splain.
Receiver of pistol (or numbered part as deemed "receiver or frame") is logged at factory as handgun-worthy. It then goes on to enjoy a life as a handgun, but can in fact become a rifle as long as it abides for rules as such, or goes NFA with proper stamps paid.
That's why you can see a Remington XP-100 receiver at a benchrest match, made into a bughole bolt rifle.
That's also why you can have an AR pistol converted into a rifle. No big deal, as long as it abides by the rules concerning rifles, no NFA configurations unless you want to do the paperwork.
However, and here's the sticky part, if that receiver left the factory and was logged as rifle-worthy, then it stays a rifle, or you go NFA again with proper SBR paperwork, etc. No handgun conversions allowed.
Handgun>Rifle>Handgun = OK.
Rifle>Handgun = Not OK.
Thompson-Center ran into this wicket with their Contenders some years back. They won.
However, the bubba who shows pictures on Intarweb forums of Mausers or Mosin-Nagants made into handguns? He's in a veritable pile of doo-doo.
Mech-Tech carbines began life as pistols, and are converted to less-concealable rifles and back again, no problem.
Regarding your other post about Kalifornia not allowing Mech-Tech carbines, that's a Kalifornia thang...
