Hatsan .25 cal AT44S-10 Long Mods

[rsterne]

This morning I had the opportunity to plug the remaining two side holes and drill and chamfer the top hole.... Here is what the top of the valve looks like now....



I added some black ink around the hole to make it more visible.... I milled the top hole out to 5/32" and then moved the mill each way 0.006" to enlarge the hole to 0.170" (the same size as the transfer port).... I then plugged the two remaining side holes with 6-32 x 3/16" SHSSs, got out the Dremel with a 1/8" ball grinder tip and smoothed off the front corner of the hole, plus the two side screws where they protruded into the (now larger) top hole.... Once I was finished, I could see the white plastic valve seat, which was "around the corner" before.... The total area of the larger single hole is the same as the three smaller holes from the previous test.... If I gain any performance (or efficiency), it will be from improved flow of the single, streamlined corner and/or the lower drag of a single larger "pipe" compared to three smaller ones.... Assuming this mod works as intended, I will add some JB Weld between the 5 set screws in the groove to get rid of the last bit of wasted volume I can....

Bob

[rsterne]

Thanks for the accolades, guys!.... much appreciated.... I'm pretty sure with a lighter pellet it would go supsersonic, but really, who cares.... Baracudas in the mid 900s are plently, and I'll likely have to detune it a bit if it's preferred fodder is the JSB Kings.... I wonder how long it will be before JSB bring out a heavier pellet in .25 cal?.... You know, something I can't figure out.... This gun is NOT that loud compared to others I have at comparable power levels.... maybe the air splitter?... EDIT: Nawwwwwww.... I figured it out.... my other guns are all running lower pressures.... therefore a longer valve opening and a higher residual pressure at the muzzle.... No magic there I guess....

One thing I noticed is that with improving the flow, particularly with opening up the valve throat.... the bell-curves are becoming "peakier".... I've seen that before, but the one that surprised me was the Kings at 5 turns preload.... The fill pressure dropped to 170 bar (normal) but there were still only 15 shots within 4%.... I would have expected more as the gun was "detuned".... I'm thinking that I may have to restrict the transfer port slightly to get to my 18 shots I want.... Maybe just putting a stock transfer port back in will be enough?.... I won't do anything until next week when I fill the groove in the valve with JB Weld though....

Bob

[rsterne]

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