These been around for a bit.
"Salamander M4 Mag adapter."
TM 870 on hpa with flash mag. Because 6 shots.
TM M870 w/t HPA & Flash Mag: http://youtu.be/m57vKz9L8-8
Was debating on getting on for my breacher or springer at some point, because yes- it would save me a lot of money, considering 2 TM shells are $18 USD before shipping. I opted out for some cheaper UTG shells and see how they'll hold up (6 pack for $18).
I could see how it be ideal for a springer, not so much a gas shotgun, unless you of course, HPA tap it, then you can just spam 6 shots on your TM and eat everyone alive.
Neat idea, kinda degrades the classic shotgun image, but for me, I prefer loading in the shells, #classy.