Since it isn't a .50 or .54- it's got to be a .45, doesn't it? I don't think they made that pistol in .36 - or did they? Anyway, big difference between .45 and .36. If the 1/2 bit shank won't go in, but a 7/16" will, it's got a .440 to .450" bore. I'd ask Pedersoli what cailbres they made in that design. Afterall, it's stamped by them. Perhaps the calibre is stamped on the bottom flat? Your calipers should give an exact measurement of the land to land, and groove to groove dimensions to without a thou or so - good enough. In .45 cal pistols, sometimes it's easier and more accurate to use a .440" ball and .018" to .020" patch. My .45 pistol, a flintlock, will cut a tiny hole for 5 shots at 25 yards off bags with a mere 20 or 25gr. of 3f, a .440" ball and .020" denim patch. It's rifling twist is 18".