I'm not positive on where to find the law (online), but I recently register my AK as a pistol. The gun must have a pistol grip, the gun in overall length must be between 26' to 30'.
I just removed my buttstock. Once I get some money I plan on putting a folding understock on it. Since I have my CPL, all I had to do was fill out a RI-60 form, and sell the gun to myself (for free). It was that easy.