Stats-wise, just start with the H&K G11 (MAG, pages 67 and 93) and add a KAC Masterkey (MAG Expansion
, pages 9 and 13).
The Masterkey is cut down to a 3-round magazine, but I suppose you could swap out the tube for one from an unmodified Remington 870. Also, you're gonna need to fabricate a custom mounting bracket for it, as it's not set up for underbarrel mounting on a G11 (and I don't know that anything is except maybe a prototype HK69 variant somewhere in the bowels of H&K's vaults). But those are mainly story obstacles, not stat issues.