Miroku's of that era did NOT follow Browning's choke markings at all, though they do look similar, they do NOT have the same meaning.
Suggest that you simply shoot it on a grease plate w/the ammunition intended for a specific use first. Then you can decide whether or not you wish it opened further.. or simply go shoot some skeet w/it low gun. Are you hitting the targets? Are the breaks good or..? Back at the grease plate do both bbl.'s seem to shoot to the same POA [Point Of Aim]? If the answers are 'yes', you are home safe; don't make further ado of it.
I'd made a similar suggestion once to another person who'd inquired here and was promptly roasted by an individual in IL. Another member got a decent pheasant hunt out of it & they both had a grand time as I recall and it did not invalidate wot I'd suggested in spite of remarks made to the contrary.
Its a quick & dirty method to find out how the gun shoots for you. I'll stick to my guns [Ha!] on the suggestion.
