Ian, I certainly "buy" that a rough bore might leak more gas than a smooth bore, resulting in lower velocity. Could be, also, that there would be a more macro friction than I was thinking of in the previous statement. I'm always glad to see data from you and to factor it into my thinking. Theory is fine, actually very handy when we have nothing else or need to explain what we have. I'd love to see your real data (don.amos@conti-na.com). BTW, I, for one, don't care where your formal education stopped. I know for sure that you turned your life experience into a fine and useful education.