If I understand you correctly, you are using a comression ferrule with a 1/4" ID for the transfer port?.... I agree that the 3/16" width is correct for a .25 cal to prevent loading problems, but I see no reason why you can't elongate the barrel port to flush with the inside of the ferrule front and back (ie 1/4" long.... You may even be able to angle the front edge of the barrel port forward to increase the area to nearly the same as the 1/4" ID ferrule.... My .257 Hayabusa has a barrel port that measures 3/16" x 5/16" at the boreline, tapering to a 1/4" diameter at the transfer port seat.... My transfer port is a piece of 3/8" Teflon rod drilled to 1/4" ID....Bob
Most if this is over my head. Let me ask this... if all other factors are the same, am I using the same amount of air whether my port is constricted or whether its opened up wide? In other words, if my gun is using 50psi per shot with its factory port, will it still be using 50 psi if I open the port up?
The down side is I have to dry fire the gun about 10 times to get there.