Estimated volume... ?
A 20" long .25 cal barrel is 16 cc, so at 25%, a dump valve of 4 cc should be a good balance between power and number of pumps required in this application.... You could get more power by using a valve that is 1/3 of the barrel volume (~5.3 cc) but it would be so long that you lose way too much pump stroke, and the number of strokes will increase dramatically, filling a large valve from a small pump....Bob
well I got 630 fps with 20.1g FTT with ~.13 ci valve and a 19" in barrel so we know you will whoop on that...
Try a Disco hammer spring....Bob