I've asked a bunch of people all with various answers and good reasons on both sides of the argument.
Can you start off with a Shockwave, turn it into a full length shotgun, and then go back to the original shockwave length as long as you don't have a <18" barrel on it at the same time as a stock? I know you can do pistol->rifle->pistol without issue (per T/C case and ATF rule). It seems like the same would hold true for the shockwave but ATF sometimes plays stupid games with wording and the Shockwave isn't a pistol, it's a "firearm".