way off topic but for the single shot,,,,,

[riuma]

See if MGM would make a barrel.
The 5mm barrel, it being chambered in that and not a rechamber. The barrel and or the lug is probably machined to center the firing pin to the rimfire pin to that particular cartridge.
I have seen this done for the Encore where it only has one firing pin. Looks way better and easier than TC’s answer by turning the OD of the barrel off center to align them.

The G1 Contender was offered in 5mm Remington Rimfire Magnum and it is not machined off center. I shoot one of these guns with a 5mm barrel. I shoot 5mm Remington RFM and the 5mm Craig wildcat: a CF version machined from Hornet cases. I simply switch from RF to CF on my G1 hammer selector and go. Best of both worlds.~Riuma