Hatsan .25 cal AT44S-10 Long Mods

[EIEIO]

That's pretty good.. Thanks for doing the work tuning the at44, I plan on copying your work if you don't mind. I started with the valve stem (.090) and valve seat I drilled out a bit more.
Thanks again
Bob

[rsterne]

I pulled the gun apart tonight and fixed the problem of the hammer preload screw moving.... I drilled a 1/8" hole about half way through and drove in a Delrin pin and then shaved it off flush with the top of the threads and screwed it in.... It was a bit too tight, but a little shaving with a razor blade and now it's nice and snug but you can adjust it when you want to.... Here is a photo of the hammer and the two allen keys to adjust and hold it.... There is a spacer between the screw and the spring, and it and the hammer spring are sitting on the long allen key I made....



I also pulled out the brass sleeve that the valve screws into and checked to see how it lined up with the holes in the valve.... It was just a fraction off lining up with one of the holes, so I machined 0.001" off the end so that when screwed in, one hole lines up perfectly with the transfer port.... The sleeve barely comes flush with the end of the main tube, so if you had to machine off more (0.010" would index the valve about 1 hole) you might also have to machine a small amount off the end of the main tube as well.... I added a punch mark to identify the top hole in the valve.... I also noted that the annular groove in the valve is narrower than the transfer port where the circle is on the photo....



Previous experimenting by others on the AT44 showed that plugging the lower three holes has little affect on velocity and reduces air use, thereby increasing efficiency.... I disassembled the valve, tapped the bottom three holes 6-32 (no drilling needed, they are the right size) and installed 3/16" long SHSSs flush with the outer (and inner) surface of the valve.... This not only blocks the holes, but helps reduce the volume of the groove.... If it proves to be the right thing to do, I will fill the remainder of the groove between the screws with JB weld to further reduce the volume....



I put a slight bevel on the edges of the groove to fair it into the transfer port.... I will likely be drilling out the top hole to the same size as the TP (0.172") once I have tested this stage.... I may also be plugging the remaining two side holes as well.... that proved to be the best, when used in conjuction with the larger top hole in the .22 cal I worked on....



The next step is to reassamble the rifle and test it with the lower 3 holes plugged to check the velocity and efficiency, and what hammer preload adjustment has to be made to compensate....

Note to Sean RE the valve threads.... The OD of the threads is 1.080" and the OD of the main tube in that area is 1.202" so the maximum wall thickness above the threads is 0.061" (and it's probably a whisker smaller than that).... The very end of the tube where the O-ring seals is 1.150" ID, so the wall thickness above the O-ring is only 0.026".... I don't even want to think about that.... I guess it works because it's a very short length at that size?....

Bob

[SeanMP]

Holy F***

You've got an off the shelf 25 cal flirting with the transonic barrier. NICE!

And enough power to blow marmots into next week

[Gippeto]

Holy F***

You've got an off the shelf 25 cal flirting with the transonic barrier. NICE!

And enough power to blow marmots into next week

I just cannot put it any better....X2

Bloody well done Bob!

Al

[SeanMP]

70 fpe AND 1.28fpe/ci

Who says there is no gold at the end of a rainbow.....BRAVO!

[rsterne]

Got to run some strings over the Chrony tonight.... can you see the smirk on my face?.... Drilling out the valve throat and slimming the stem, combined with opening up the inlet on the valve.... gave me the highest performance I have yet achieved.... The larger valve seat required an additional turn on the hammer spring preload to overcome the extra force holdind the valve shut.... but even without doing that the performance was up, although it was at a lower pressure.... Once I went from 6 turns preload to 7, the entire 17 shot string of 25.4 gr. JSBs was over 1000 fps, peaking at 1044 (61.5 FPE)....



I didn't bother adding more hammer preload when I shot the Baracudas and EJ Domes, so the graphs above are a direct comparison of the three pellets at the same power setting.... The 'Cudas peaked at 65.4 FPE and the EunJins at 67.5 FPE, but they would be shooting over 70 FPE had I bothered to add enough preload to bring the start of the string up to 200 bar instead of 175.... I cranked up the preload for a few shots with the EunJin Pointed 41.7 gr. pellets, and at 11 turns of preload and 190 bar I got 78.4 FPE.... I'm thinking with 12 turns, right at 200 bar, I can hit 80 FPE (for one shot)....

This rifle has turned out to be a real beast.... I've likely got too much velocity even for the 'Cudas, but until I get the opportunity to put some lead a lot further downrange I won't know if I can get away with leaving the tune as is, or if I'll have to detune it for accuracy.... Nice problem to have, though !!!

EDIT: I forgot to add that the efficiency for the above strings was 1.11 FPE/CI for the Kings, 1.21 FPE/CI for the 'Cudas, and 1.28 FPE/CI for the Domes.... so not only is the performance up, so is the efficiency....

Bob

[rsterne]

I suggest you get a Chrony.... trying to duplicate somebody else's results is almost impossible....

Bob

[bruce32466]

Thank you Bob for being so quick.
You're right about the chrony but what do you think of the regulator? do you have a prefrance for any one regulator?
Thanks.

[SeanMP]

Note to Sean RE the valve threads.... The OD of the threads is 1.080" and the OD of the main tube in that area is 1.202" so the maximum wall thickness above the threads is 0.061" (and it's probably a whisker smaller than that).... The very end of the tube where the O-ring seals is 1.150" ID, so the wall thickness above the O-ring is only 0.026".... I don't even want to think about that.... I guess it works because it's a very short length at that size?....

Your right that would creep me out big time. I did the calculation though at the low end of CrM and your still looking at 4300psi burst. And if the Oring completely fills this reduced area then more than half the force is being taken by the plug. Hehehe I wouldn't do it but theoretically its fine.

That valve mod makes perfect sense now that I see it in action. With that much void space it would induce those little orifices to instantly hit choked flow because of the sudden pressure drop.

[rsterne]

I got some time to shoot some strings last night to test the plugging of the three lower holes.... I really like my "Chrony Connect" program and cables I got from the UK, it saves me having to stop and record every shot.... The velocities are stored in a file that I can copy into Excel.... I shot three strings with the hammer preload adjusted 6, 7, and 8 turns out from full in.... The new plastic locking plug in the adjusting screw works perfectly, NO sign of any creep in the adjustment any more.... Here are the results.... NOTE: pressures on the graph are in bar, not psi....



The first thing I noticed was how much smoother the curves are.... They don't have the large shot-to-shot jumps I saw previously.... It requires two turns more preload on the spring to get the same average velocities as with all six holes open.... but the peak velocity occurs at a slightly higher pressure than before, making a more even bell-curve rather than one with a long front end and quick fall-off after the peak.... This is typical of what I expect to see on restricting the flow slightly.... I also achieved an increase in the efficiency, ie slightly less air was used to generate the same FPE.... 7 turns out gave me 23 shots with 4% averaging 898 fps (45.5 FPE) filling to 200 bar and shooting down to 140, with an efficiency of 1.20 FPE/CI.... That's already a pretty potent quarter-bore....

I'm happy with these results, as they are consistent with what I saw on the .22 cal I worked on.... The bigger caliber required two turns more preload to get back to the same power, whereas the .22 cal only needed one.... I think all we are seeing there is the need for more airflow in the larger caliber.... The next step is to plug the remaining two holes and drill out the top hole to match the transfer port size....

Bob