P-12 with HK UMP 45
The P-12 would be limited to 2 3/4'' shells for pump operated fire, but you can hand load and fire 3" magnums. Ejection is said to be from the bottom. The Halo Mfg website does not have any new info on the P-12 and it was orginally de to be released in 2008, but I certainly haven't seen one. Rumor has it that the design is currently being refined by Monolith Arms. I would love to see it come to market and offer some competition to the Kel-Tec KSG. I would like to see a muzzle brake of some kind added, maybe just a standoff breacher-type, because recoil is likely noticeable in a bullpup 12 gauge firing slugs. A nice beefy rubber recoil pad would not be out of place either.
P-12 with FN P-90
Whether or not we will see this design come to market remains to be seen. Certainly there is plenty of vaporware out there these days, seemingly invented just to torment us.
(I am looking at you Kel-Tec PDW. )