I think it's probably a safe assumption that the groove diameter of your barrel will be around .429", but until you find out the throat diameters of the cylinders, we're all just guessing as to why you can't get the accuracy you want. Finding the diameter is as simple as seeing if a bullet of know diameter will drop through the chambers, or even removing the cylinder, placing it on a soft workbench top, and driving a lead bullet through one of the chambers, then measuring it with a micrometer.
