As long as the barrel is a minimum of 18" and the OAL is a minimum of 26", it's legal. On a single-shot like the Pardner, the barrel is obviously going to have to be longer than 18" to get you to the 26" OAL minimum. Aftermarket pistol grips for just about every shotgun under the sun have been offered forever. As long as any shotgun configuration meets both (not either/or) of the above minimums, it's legal in the eyes of the BATF. (And, of course, most people like to add a 1/2" or so all around just to be sure their gun is compliant).