It looks like the 12 and 20 are the same as per Axe.
The Remington Tac-14 20 gauge, is on a smaller receiver than their 12 gauge Tac-14. I have one of each, and like the 20, but the stocks are the same as the 12. The pistol grip uses an adapter to make the larger grip fit the smaller frame, and it looks like the Magpul fore stock is also 12 gauge size. I don't have either of the Mossberg Shockwaves, so I can't help with that.
Seeing no use for the Tac-14, even though I have bought five of them, but I sure do like them. And I just might buy another one to give the finger to the NFA.
My hobbies are felonies in California, and soon to be in Virginia.