pyrodex was meant to be an alternative to black powder for use in muzzle loaders . I have decided to obtain a muzzle loaded shot-gun and go clay pigeon shooting with it . The route to legal aquisition is simple
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1 book a short clay pigeon shooting morning to make sure you wish to participate.
2 attend a safety and handling approved course on shot gun use at local club or shooting ground
3 now that you can demonstrate competence and safe handling apply for shotgun certificate and license for storage of black powder.
There are lots of regs re safe storage etc that i have not listed as they are already publicly available