One thing which has puzzled me about this gun is that when I dial the power back with the hammer spring preload, the shot count doesn't increase.... I decided today to fit the oversized 0.153" non-PAL valve stem into the now very much oversized ports and test it.... After doing that, I turned the stem down to 0.140" in the throat area, which is the size of the main part of the stem, and ran more tests.... Here are the results....



It appears that the best way to tune the Hatsan is to adjust the valve stem size to adjust the power, and then use the hammer spring preload to adjust the operating pressures.... certainly if you want to get the best efficiency.... Using the best strings from the above and previous graphs, and always starting from 200 bar (or as close as possible), here are the choices I have so far with JSB King 25.4 gr. pellets.... In all cases, only those shots within 4% of the maximum velocity in the string are counted, and the average FPE for the string is stated....

17 shots at 59.5 FPE (1027 fps) at 1.11 FPE/CI, refill at 130 bar
19 shots at 55.4 FPE (991 fps) at 1.13 FPE/CI, refill at 133 bar
22 shots at 51.6 FPE (957 fps) at 1.16 FPE/CI, refill at 130 bar
25 shots at 45.6 FPE (899 fps) at 1.22 FPE/CI, refill at 133 bar
31 shots at 43.4 FPE (877 fps) at 1.28 FPE/CI, refill at 122 bar

Other than with the mildest tune, I have a usuable pressure range of 1000 psi, from 2900 down to 1900, and with the 43 FPE tune, that range is extended down to about 1750 psi.... Other tunes in between should be possible with the right combination of valve stem diameter and hammer preload.... Small adjustments can be made with just the preload.... I'm very pleased with the shot count and the efficiency figures I have been able to achieve with this rifle.... and I'm looking forward to doing some accuracy and pellet testing....

Bob