Robert the valve should only stay open the same length of time allowing the same amount of air out each time, but this is not a perfect world and you can see that there are slight variations in pellet speed, but is that from slightly more air being passed through or because of a slightly lighter pellet. Other than being controlled by springs to close the valve a pellet weight should not have any effect on how long the valve stays open,I am sure there are many views on this but from a practical point of view pellets should not have any effect on how much air the valve allows through each time it is opened, that is why shooters increase power to shoot heavier pellets.
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