Currently for sale on our Trade Me auction site are lead bullets for the 577 Snider and newly made 577/450 cases. The seller occasionally advertises new 577 Snider cases too as shown in the image with the lead bullets.
The cases are formed from 12 gauge brass shotgun cases.

From the seller - "This auction is for 20x .575 lead 'minie' projectiles. These are great for loading into newly sized snider brass or to go into your .577 muzzle loader as the minie base, or skirt,
is designed to expand and tightly fit the rifling when fired.

Made from pure soft lead. These are good option for fire forming snider brass before using a larger bullet such as the .600 flat base projectile I also stock."