I know this is an old topic but see it first time now. Marakai can you make a drawing from the 20/577 henry. some years ago i have asked NDFS for making a die set, have thinking the brits may know it but they cant help.I like only to have a die to make collector cartridges, dont have a gun. Griffin and Howe in the USA have a 20/577 double on their website, believe for 12000$$$. Are you interested in 20gauge 2,75" brass, this was made in russia and is probaly allways making. this brass use the 6,45mm berdan primer and its stronger than magtech shotgun brass. I looking for this some time, get only collectors pieces till now,and it may have a much better price than your 600/577 brass. pay for collector stuff 2-3 euro.tell me if you have an interest.

duggaboy, your capegun is not making for the 58 berdan carbine! this cartridge was never available in germany. there exist a rifle cartridge family making with shotgun cases from 36 bore up to 12 bore.you find chambers for 33mm, 40mm, 50 mm and 65mm long cases. this is not a british paradox or full rifled cartridge. the most common is the 40mm long brass case in 28, 24 and 20 ga.
this cartridge is allways a handload, havent found till now a factory load. it shots a special lead bullet: the so named cats head bullet and we have pics in an old RWS bullet catalog.Seems to be that your gun is chambered for the 24/33LK cartridge.
I search for this cartridges and gun for many times but infos are very rare, have and shot a 20/65 - 28/40LK( LK means lancester Kugel in english lancester bullet) grove dia. is .600".I make my brass from 28 ga. Magtech brass and you can make yours from 24 ga. Magtech this is cheaper than 577 NE. nice to hear chamber dia. also barrel dia. from your gun