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Perhaps a 600 bpe?

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There wasn't one...

...it was the 20-bore BP rifle cartridge, brass case!

If it chamber-casts .600 NE then groove diameter might be .615 to .620?

24-bore should be groove diameter .577 to .580

Not much help, sorry.

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listen and learn

german combination guns have the rifle barrel right, british combination guns have the rifle barrel left so they make it many times different.

british bore rifles have the shotgun caliber as groove diameter so maybe .577 -.580 bullet

BUT

german bore rifes have the shotgun caliber as lands diameter so .620 bullet

THIS IS COMPLETELY DIFFERENT

if the Teschner Collath have a .597 groove diameter I would say .600 is fine