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Great ible. I made mine the same way except I domed the nickel using a ball pein hammer, large socket and the rounded end of the vise handle.
Just an FYI. Paint stirrers are perfect for this. Even comes with a handle
Just an FYI, GB/VX will penetrate duct tape with liquid exposure fairly quickly and after a short time in aerosol. The military uses a special tape to seal gloves and boots to the level 2 suits, not duct tape, for this very reason. Mustard gas is a misnomer, mustard agent is more of a contact hazard than an inhalant. It should work OK for the other stuff, if it is airtight.
What are you using to hold the shell casing to the chuck?