There's a discussion about plastic shells here. Seems they're not suitable. reloading plastic 24 gauge shot shells for snider
I have a .pdf how-to for making a reloadable .577/450 case out of 3/4" brass bar stock if you want it. Basically turns the rifle into a .45-70. PM me your E-mail.
"...require a large press that will accept 1 1/4" dies..." This is important.