Carbine conversion of an MPA Defender pistol

[cluttonfred]

OK, as I understand the rules, as long as you never combine the shoulder stock and the short barrel, conversion of a gun that started as a pistol to a carbine with shoulder stock and 16" barrel and back again, like a Thompson Center Contender or Mech Tech Glock conversion, is allowed. (See https://www.atf.gov/files/regulations-rulings/rulings/atf-rulings/atf-ruling-2011-4.pdf)

I found a source for the basic Masterpiece Arms MAC-style, top-cocking Defender series 9mm pistols for well under $400. The pistol might be fun as a range toy but doesn't seem very practical otherwise. With a 16" barrel and a shoulder stock, however, it looks like it would make a neat little carbine, though with a very short sight radius.

Are there any pitfalls or legal issues that I am missing here?

 

[USAF_Vet]

Shoulder stock, 16"+ barrel, and 26"+ over all length is a rifle, regardless of the type of ammunition it shoots.

If it started life as a pistol (manufacturers records can prove this) then yes, you can install a longer barrel and a stock, converting it to a rifle (26" OAL). You can convert it back to a pistol at any time.

If at any time it has a shoulder stock and a barrel less than 16", it is an unregistered Short Barrel Rifle (SBR) and that is illegal. I don't know if there is any leeway granted of the weapon configuration during the assembly that may temporarily create an unregistered SBR, but it's a good habit to install the barrel first, stock last. Especially if you are in the habit of assembling guns in front of ATF agents.

No pitfalls or legal hurdles I can see. It'd be like a full stocked, long barrel UZI, fun while less than practical.