We have a lot of data that shows chamber reducers "work." That is they launch shot loads at near enough to expected/required and with good enough patterns to be relatively widely used. We can check our assumptions to some extent against that fact. If there were no blow-by and no pressure reduction due to bore volume increase, there would be a velocity increase. A large pressure loss due to either blow-by or bore volume increase would lead to a lower velocity. We may have a small increase or decrease in velocity, so we have to believe that pressure changes are relatively small.