I've followed this thread with interest.

I'm not telling you all how to do it, I realised long ago, it's impossible to tell people how to do it. I'll just tell you how I do it.

How far are you hoping to fire your Snider at? 50 yds with a .600 round ball? 100 yds with a .577 slug? 200 or 300yds with a .590 slug? any further I'm wasting my time. A Snider isn't accurate enough to hit an NRA Target any further. Therefore a full military load isn't needed. 50 gr of Tpph is plenty.

Magtech brass resized, shortened, or whatever is near useless. The internal capacity is too great. Bite the bullet so to speak and buy Bertram from Kranks. You'll still need a cookie or lubed wad behind the slug with Bertram, to take up the air space.

Forget round ball of any description, above 50 yds they're wildly inacurate. Buy a Lee .577 mould. Stick it in a four jaw chuck, aluminium is very easy to turn. Grind a simple tool to fit the lube grooves, and open the mould to around .590 or .595. Trial and error will dictate the size for your Snider

Turn a basic steel mandrel, to fit into the ram of your press, to flare the bertram brass to .590/.595.

Visit your local nut, bolt and fastening retailer, relieve him of his 7/8" unf 14tpi bolts, he'll be glad to see the back of them coz he's all metric now. Cut the heads off with an Hacksaw. Stick 'em in a three jaw chuck drill down the length, then bore out to whatever size you need to size your cases down to your slugs after shooting. A bit more trial and error is needed here also.

Job done.
A semi wadcutter slug, seated in reasonable brass, in front of a sensible charge, which doesn't punish you or rattle your antique rifle to bit's, and will land in the black at 200yds every time, with the occasional bull as a bonus.