Why would it be illegal to put a longer barrel on a pistol? is it illegal to put a longer than 16" barrel on an AR pistol? I don't think its illegal, maybe not the best handling pistol in the world, but I don't see how it would be against any ATFE regs.
Now if he were talking about putting the charger barrel on a 10/22 rifle receiver with a stock, completely different ballgame.