Can you turn a pin into the front of the hammer, slightly less in diameter than the valve stem?.... or insert such a pin into the front of the hammer?.... That should allow you to push the valve stem just past flush with the back of the valve.... Maybe?....Bob
I run a .357 cal valve in a .224 with no issues, I just use a tapered transfer port.... Tunability remains fine.... Large throat and poppet doesn't seem to bother it one bit.... Mind you, it's a balanced valve, not a Cobra type....Bob
it seems to require much more air to generate 160+fpe on 160cc of plenum/340cc res vs a 500cc unregulated tube.
That would seem to be excellent efficiency.... Your HP reservoir is 340 cc, correct?....Bob
If the last 2 shots drop the pressure to 2400 psi, the additional air used would be....(200 / 14.5) x (500 / 16.4) = 13.8 bar x 30.5 CI = 421 std. CI.... Add that to the 1285 used above the setpoint and you get 1706.... and the efficiency drops to (1988 / 1706) = 1.17 FPE/CI.... which is still pretty decent for 160 FPE in a .257.... I am very surprised that 0.220" of travel on a 0.275" throat (80%) was not enough to find a good tune.... Most of the time I only allow for valve stem travel of 2/3 of the throat diameter and never find the hammer hitting the back of the valve.... Efficient tunes often occur when the valve lift is only about 1/4 of the diameter, and never in my experience more than 1/2 the diameter....Having a lighter hammer than OEM, you will need more spring energy to get the FPE you want.... and it will be accompanied by more lift to get the same dwell.... Opening the valve more than 1/4 of it's seat diameter doesn't increase the flow rate, so all gains in power must be made through dwell.... The dynamics of the lift to dwell are governed by the hammer mass, so a heavier hammer would give you the same dwell with less lift.... Perhaps that is the reason your hammer is hitting the back of the valve.... I would try the stock hammer again....For an unbalanced valve (and the Cobra uses an air spring, it is not a balanced valve), my rule of thumb for hammers is the weight in grams times the travel in inches gives the FPE you can reasonably expect the gun to generate.... My unregulated .257 Hayabusa, running on 3000 psi with a conventional valve, used a 113 gr. hammer with 1.20" of travel (113 x 1.20) = 136 FPE.... It peaked at 160 FPE, and delivered a good, efficient tune of 135 FPE, just about where you are at.... I would get 12 shots within a 4% ES from 2960 psi down to 2280 on a 28 CI reservoir, for an efficiency of 1.13 FPE/CI.... It used bore-size porting, but did have a 0.088" bolt probe, which dropped the smallest port equivalent down to 0.241"....With a balanced valve in the same gun, I am now using a 77 gr. hammer with 1.09" of travel reduced by an SSG gap of 0.150" at my current tune, so the actual (powered) hammer travel is only 0.94".... Multiplying those together only give (77 x 0.94) = 72 FPE (half the hammer strike), and yet the gun still delivers 136 FPE and with better efficiency at 1.30 FPE/CI... and with a lighter hammer spring to boot.... Bob