Cutting down spent shotgun shells

I got a request to make a shotgun shell ring in a smaller size than what was available in my Etsy shop, and the request included the possibility of taking down the height of the setting.  When I read that part of the request, I was on board - I wanted to make the settings lower on the original rings that I made but couldn't get past the material in the base of the spent shotgun shell.  It's a metal casing filled with a thick, solid plastic material and a center metal rivet.  So I set out to try to cut it down again, hoping for better results.

I tried using every type of flexshaft bit I had that I thought might cut through.  I got past the metal, but the plastic in the center just kept gumming up. I was thinking that I probably needed a different tool or set of bits...what I have is for metal and I needed to get through the plastic.  So I did a trillion web searches and found information on saws and rotozips and cut off wheels (cut off wheel: tried it.  Nope).  Then on search one trillion and one I found a mention of melting the plastic out instead of cutting through it...either through boiling or with a torch.

I wasn't about to try to boil those suckers.  I cannot even imagine the horrible mess that would make and I don't cross the line between jewelry workshop and food area.  So I tried heating the base and plastic of the spent shotgun shell and it WORKED!!  I found myself looking around to see if my imaginary audience was as surprised and delighted as I was...all that searching and cutting and general wasted effort and all I had to do was just melt the plastic out.  It literally took 30 seconds.

The other rings I made were fairly tall:

This one ended up being about half as high.