Looks like I may be proceeding on this build as I just purchased an 1889 with fluid steel barrels in 12 gauge that is in decent mechanical condition. The barrels are badly pitted so I believe they may be a good candidate for a mono sleeve conversion to 20 bore. I can order 20 bore (.620) barrels with any twist and rifling depth I desire. I can also get them in "certified" barrel steel. I plan on shooting a short slug or grease grooved bullet loaded in brass 20 gauge cases. Does anyone have any suggestions on the rate of twist and depth of rifling that would work in this case? Is there any way to calculate bullet weight by it dimensions? Thanks for any input,,,,Ed