I had a little time to play this morning. Here's where I'm at.
.357 mag brass. 2gn 700X, over powder cardboard wad, .380 case of rice, capsule of rice.
I expect a .38 case would do just as well with exactly the same procedure and amount of rice. It would just require a minor adjustment of the "seat/crimp" die.
When I was crimping my loads with over shot wads, it always wanted to just bend one side. Which was fine for those, but I figured it would probably give me pretty bad runout with the capsules. That may or may not be a problem, but I figured I'd just head that one off at the pass. To that end, I reversed the shaft of the decapping pin on my .300BLK sizing die to make it be a seater/crimper.
https://i.imgur.com/r6tqUK6.jpeg
Here's a closer shot of a couple loaded brass. You can see where a shoulder is just barely being pushed on.
https://i.imgur.com/U39f3nt.jpeg
These are in there SOLID. You are not pulling it back out with your bare hands.
Results: (I've marked each standard sized printer paper with the distance in feet.)
https://i.imgur.com/n6lzxvK.jpeg
https://i.imgur.com/vIIj9KA.jpeg
https://i.imgur.com/t1Lu6zt.jpeg
https://i.imgur.com/nvxeLkR.jpeg
Either the crimp results in enough pressure, or the "seater" weakens the front of the capsule enough that I don't have to score it to get it to break open.
It does build enough pressure to expand the case mouth such that I can seat a new capsule without having to expand the case mouth. The small end of the capsule will drop in but the fatter end requires some force.
I did have 1 of the 5 cases split. But, I have no idea how many times this brass has been fired. So, that may or may not be a thing. The fact that I don't have to expand them to reload should keep the splits to a minimum (I think) as the amount of sizing taking place the 2nd time around is minimal.
On the right is one that I fired and shoved a new capsule in and crimped it.
https://i.imgur.com/K6hTotk.jpeg
I'm out of time for now. I plan to do some lead shot testing soon.
Please steal and mutate the ideas I have stolen and mutated.
I think we're on a good path here. Speer might even lose some shot capsule sales over it. ;)