Howdy to the camp,
Well I built a prototype shot shell, though it is a lot of work for one shell. It chambers but does not feed (a little too long). I have not fired it to see pattern yet.
I used a Starline 56-50 case. Thinking a 56-50 shot shell would be close to a .410, I took a 2 1/2" 410 shell, took the powder, and put it in the 56-50 case. I placed a cardboard wad over the powder. For the shot capsule, I used the plastic case that a 3cc syringe comes in. We have a breeding kennel, and buy shots and vacine by the case. The end where the needle cover projects is just under .5" and it tapers open towards the plunger end. I cut the needle end flush (there was a protruding collar with a hole in it) and cut it to about 3/4" long. I seated and crimped this in the case sitting on the powder wad. I then took the 1/2 oz of #6 shot from the 410, and poured it in the open end of the capsule. I placed one of the paper wads from the 410 on top of the shot, and sealed it with a drop of gorilla glue.
It looks good, and should work, but it is just a little too long to cycle. SIngle loaded, however, it would be a close range 410!
Personally, as I said, it was a lot of work. I think the 348 Winchester brass may be the answer.