I've never loaded shot cartridges for a revolver, but have watched my dad do it and talked to others that have also.
Best results are had with 800-900fps, or even less velocity.
Yeah, yeah - right off that limits effective range, but fighting the centrifical force of the rifling ..... WHY FORCE IT?
I would use #8 shot or no larger than #7 1/2.
Two ideas come to my thinking;
1). Using Speer shot capsules, after charging the Maxi case, seat the over-power wad as deep as possible to be flush on the few grains of powder in the case.
Pour the shot pellets directly into the casing, but only enough so room is left to push the plastic capsule full of pellets (minus the OP base) into the casing.
Of course you'll have to juggle pellets already in the casing with all the pellets in the (open ended) capsule -
trial and error obviously! Once you figure out how to get all pellets into this special load and the capsule to the correct depth, a very light crimp I believe is called for,
but not heavy enough to crack the plastic capsule!
2).
Another method to make a Maxi shot load might could use a gas check flipped upside down to act as an over-powder wad, and if you have a plastic capsule load it full of pellets like I tried to describe above.
OR ..... Use an upside down gas check as an over-powder "wad," fill the case to the top with shot and then put another upside down gas check on top, and crimping it closed! (like a wadcutter or similar).
I wonder if effective range would be further than 8-10-12 yards, but for special survival rounds or grouse-potting rounds,
I'd take what I could get! In a scenario like originally described above, I would prefer packing a T/C Contender with a .410/.45C barrel, if not a singleshot .410 or 28 or 20 gauge!
BTW, the Contender (or G2) with a .410/.45Colt bbl works good within its limits. "Target" should be the grouse's or snake's head/neck area.