your 20 gauge barrel must have an inside dimeter between 15,70 and 16,20 mm. give it to a barrelmaker and let it rifling to a 16,75mm grove diameter. than a 3" 20 ga chamber and and you have a german bore gun.

there measurments for brass and for paper cases, the upper ring is allways 16,75mm, the lower rings measure 16,05mm for paper and 16,65mm for brass cases. the 20 ga cats head bullet itself is 20 mm long.