I've used .444 Mrlin brass for making up 1/2ounce loads in my daughter's break open .410's when they were kids.

I use gas checks as over-shot wads, crimping in a normal seating die. Worked perfectly and outpattered both 2 1/2" and 3" at 30yards with modified chokes, reamed from the factory full versions.

I didn't know a Casull would chamber, but then a .45 colt should as well. I wouldn't fire either one in a .410 myself.

We shot the 'cut case' slugs at fox and jackrabbits when we were kids hunting back in Ontario - 12 bore and 20 bore.

The tight full choke in the Baikal over and unders would usually pop the crimps and start spreading shot as well.

Accidently shot a raven with one (old weak case let go just above the brass) as it was flying over our decoys, some 10 or 12 years ago. Made quite a hole in him. I guess it was his day to depart.