This is my stock tuning experience and I think I experienced your "Double Umberella" this weekend tuning .177 box stock.I started at the top with regulator all the way in and hammer at maximum, opposite of your starting point. It produced bell curves with no regulated flat velocity string. I then began backing out (CCW) the reg screw until a flat string began to occur at 1 3/4 turns out (green string).Backed the reg out another 1/4 turn CCW and also reduced (CCW) the hammer 2 full turns Yellow String- Double Umberella maybe? definitely below the set point with some valve lock up front.One turn up on the hammer (CW) produced 24 shots @4% (Pink String)Wanting some more shots I backed out the hammer 1/2 (CCW) just below set point as it finishes with a velocity spike (minor double umbrella), but 40 shots at 4.4% spread and 10.3 FPE. I will shoot with this for a while as it shoots 812FPS with the H&N Terminators which holds a 1/2 zero to 34 yards. I think there is a meeting with the dremel in the future. I also think "Double Rainbow" is funnier than "Double Umbrella" ...
I digged up my old testing results.The first string the hammer is at maximum and produced a relatively stable short string. Syhould be fine for hunting purpose.The second string I reduced hammer by one full turn, produced the double umbrella or double rainbow. In retrospect it looks pretty good. But at that time I ticked with it more and never got back to this string.
Okay I went ahead and tried a single series Belleville stack.Each washer is 0.016" thick and 0.023" tall (meaning the travel to compress flat is only 0.007"). That means the original 5x nested stack (( )) (( )) (( is 0.039 * 5 = 0.195" tall. So to make a single series stack approximately the same height, I needed 0.195 / 0.023 = 8.5 washers so I rounded up to 9.Something else I did while I had it apart was to try an idea I threw out way back in this thread, and that was to flip over the white washer in the regulator. My thinking was the tapered screw fitting against a tapered recess in the washer is too large of a surface area to seal well (prone to creeping). By flipping the washer over, it puts the tapered screw against a crisp edge. Granted I did burnish the two together just a bit to make sure they mated cleanly but not enough to make an obvious bevel on the washer. Someone mentioned accidentally doing that and it wouldn't work but I suspected they threaded the screw into the brass piece too far and just sealed it off, so I applied a bit of threadlocker and left some room for the screw to move up and down (open and close).And well, it looks like it's working. It was getting late so I haven't fiddled around with the setpoint and hammer spring much yet but it looks like its regulating very well. I chronied 20 shots and they were all between 632 - 640fps (1.25% ES) before the onslaught of mosquitoes drove me back inside.635633636633638632634636633638640636632634640633636639635637I know this tune won't appeal to many people but 7fpe is a good place to be in a short-barreled gun as far as I'm concerned. That's close enough to the 8fpe I was wanting when I first got it a year and a half ago. One nice benefit is I was able to back off the hammer spring to the point where it just barely holds the hammer against the valve stem and that has significantly lightened the trigger pull.Not sure yet what the shot count will be but the needle is moving very slowly compared to the previous tune, as well it should.Of course I thought I had this thing licked once before and it proved me wrong so I'll report back after using it a bit.
You said "I know this tune won't appeal to many people". Why not? What's the problem with this tune?