Disco Transfer Port Size

In case you were wondering how to detune a Disco for more shots, here is some information you may find useful.... I have been able to get three sizes of ports, the stock 0.140" (9/64"), one that measures 0.109" (7/64"), and a stepped one from a Canadian 1377 that measures a tiny 0.070".... I made some intermediate sizes by drilling out some of these, ending up with additional ports measuring 0.166" (as big as you can go on a steel Crosman port, and the same size as a poly port), 0.156" (5/32"), 0.125" (1/8"), 0.094" (3/32") and also one measuring 0.081".... I used a tethered Disco running on a regulator, and tested all the ports at 1600 psi, which is the top of the sweet spot in a stock Disco.... I then reduced the regulator pressure to 1200 psi, and then to 850 psi, and tested the commercially available ports.... A stock Disco hammer spring and rear plug were used for all tests.... I used 14.3 gr. JSB Express pellets, and shot 10 shots with each combination, recording the average velocity with the following results....

http://i378.photobucket.com/albums/o...nsferPorts.jpg

You will note that increasing the transfer port size without also increasing the size of the barrel port and/or the valve porting pays no benefit.... This makes sense, as the barrel port measures only 0.134", slightly smaller than a stock transfer port, and is obviouly limiting the flow.... Reducing the diameter, however, does reduce the velocity, although the 1/8" port hardly makes any difference at all.... When you fit the 0.109" port, the gun is noticably quieter, losing about 6% in velocity and 12% in energy.... From that point on, losses are pretty linear, with the smallest port resulting in a very much quieter report, a loss of about 25% in velocity, and about 45% in energy.... Very similar losses are observed at the lower pressures, starting from a lower level with the stock (0.140") port.... It is interesting to note that the gun is louder at 1200 psi than it is at 1600 psi with a stock port, as is observed with an unregulated Disco....

I hope that the above information will be helpful to anyone contemplating detuning a Disco to increase the shot count.... I did not try weaker hammer springs, they would, of course, increase the shot count at pressures below 1600 psi.... A spring from a 22XX can be made to work at about 1200 psi, and one from a 13XX at about 1400 psi.... or an RVA which allows the preload to be reduced would be beneficial as well.... Too much hammer strike results in wasted air and excess noise, so it must be balanced against the pressure used....

Bob