Given the short barrel (12"?) how long a butt stock can you have and still be legal (without going the SBR route)?
(DISCLAIMER, I'm not a lawyer, nor did I spend last night in a Holiday Inn, so my advice is worth what you paid for it...

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Look at the Federal laws (also you particular state).
They define rifle, shotgun and handgun by barrel and overall length combinations.
How is the Mare's Leg classified now, its a handgun, isn't it?
Interpreting the law is tricky, and the only one that matters is the ATF's interpretation. Exceed that, and you could be arguing before a judge.
Basically, if you have something that functions as a stock, that is clearly intended to be used from the shoulder, and the barrel is less than 16", it is an SBR. (always, always ALWAYS get approval from ATF BEFORE making such a device!!!)
SO, how long could a Mare's Leg stock be, with a 12" barrel??
I don't know. Logic suggests that until you reach the minimum legal overall length for a rifle, then its still a handgun. BUT the ATF might have a different definition.
I suggest you ask them. And, if you actually plan on doing it, that you get their answer in writing, notarized, if possible, BEFORE you do it.