Don't know if they're available in your area, but I believe it's Mag Tec that's making brass cases for shotgun shells now. I have some for 32 ga., which is essentially just a .50 cal. They seem to be worth the cost, at about $1 ea. They're set up for priming with LP primers, of whatever make you have. They ought to do really well for you if you can find some - always a question these days, it seems.
And FWIW, when my son was very young, I got a break action .410 and took some 3" factory loads, opened them up, and put 4 .380" lead balls for cap n' ball pistols, and filled the rest up with corn meal. The primers indicated some extra pressure, but nothing I was worried about with all the metal around the breech and barrel. It was built on a 12 ga. frame. Those loads printed all 4 pellets in a space I could cover with my hand at 40 yds., which really made my eyebrows hit my hairline. I took him and that little gun with me (cut bbl. to 19" so it's balance and swing better for him then, and cut stock to fit also) deer hunting on some deer drives I went on just for him, and he was surprisingly good with that little gun. Threw some clays for him at close range with bird shot, and he hit most of them. The young will really listen if you approach them right! The little .410 lends itself to specialized ammo, I think, more than most guages do. I have no doubt that 4-ball load would have killed the biggest deer in my state! And his accuracy with it was surprising, too!