Hey there! That's a great question about 10-gauge bore sizes. It can definitely get a bit tricky with the gauge system. While I don't have the precise measurements in inches and millimeters right off the top of my head, I recall that 10-gauge shotguns typically have a bore diameter around .775 inches (19.69 mm). However, there can be slight variations between manufacturers due to different choke constrictions and manufacturing tolerances. It's always a good idea to measure your specific barrel if you need exact dimensions, especially if you're looking into custom loads or slug accuracy. Have you checked any of the SAAMI specifications or other reloading manuals? They usually have very precise data for these things. What's prompting your search for the exact measurements? Are you looking to reload, or just curious about the specs?