Converting GP11

[Parashooter]

. . .these are fitted with large rifle primers. They're 10-20 thou smaller than the original berdan primer.

Swiss Berdan primer diameter: .217"
Large Rifle Boxer primer diameter: .210"
Difference: .007"
Thickness of material required to reduce diameter by .007" = .0035"
Average human hair thickness: .004"

Here is another approach -