Back in the "good old days", an outfit by the name of Forrester or something close to that offered a percussion cap making kit. IIRC, it consisted of a form to shape the cap body out of an aluminum can, a wood rod about 3/16" in diameter and a single hole punch.
That was used to punch out dots from a roll of caps for a toy pistol.
After the body was formed, the wood rod was used to insert the punched out cap into the body. Need more umph? Put in another cap, or two.
It sounded kind of hokey to me, so I never tried it. I had lots of caps on hand, and anyway, RWS red labels were available for $0.99 a tin. Like I said, the good old days.
John (Bigsmoke)