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Thread: Hatsan .25 cal AT44S-10 Long Mods

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  1. #1
    Moderator rsterne's Avatar
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    Hatsan .25 cal AT44S-10 Long Mods

    Now that I'm starting with a "full power" AT44, I'm moving the thread over to the "PCP" section as that is more appropriate from here on.... Here is the previous thread: http://www.canadianairguns.com/showt...Long-in-25-cal

    The first mod is a relatively simple one that allows the hammer spring preload to be adjusted without removing the stock.... I didn't invent this, just repeating it here for convenience.... The hammer spring preload in the AT44 is adjusted by a screw residing inside the hammer.... It is accessed through a hole just below the safety, using a 4mm allen key about 7" long.... Grinding/filing a slot in the top of the stock allows the allen key to enter the hole.... The hammer is free to rotate, so you also need a way to prevent that.... There is a hole in the bottom of the main tube, 1/2" behind the main stock mounting bolt.... If you peer in through that hole, you will see the hammer, which has a matching hole in it.... The gun must be uncocked for the holes to line up.... Inserting a pin through both holes locks the hammer so you can adjust the preload screw.... Unscrewing the adjustment (ie CCW when viewed from the rear) INCREASES the tension.... Here are photos of the mods to the stock....





    Once the stock is back on the gun, you can insert a small allen key into the hole in the bottom of the stock until it engages in the hole in the hammer to prevent it from turning.... Grinding a taper on the end allows you to slide it in easier if the alignment of the hole in the hammer isn't perfect.... If it won't drop into the hole in the hammer, then rotate the hammer using the long 4mm allen key until it does.... Note that on the pump action, the hole in the tube is further back, and the hammer must be cocked for the locking holes to line up.... The hole in the stock must be drilled further back as well....

    On my rifle, the preload screw moved easily, but I have seen them either loctited or squashed flat to make them tamper resistant.... If the screw rotates too easily, it may be necessary to put a drop of loctite on it once you find the best setting....

    Bob
    Last edited by rsterne; Jul 28 2012 at 11:45 PM.
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  2. #2
    Moderator rsterne's Avatar
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    Here is the data for various hammer spring preloads using JSB King 25.4 gr. pellets in the "full power" version of the AT44S-10 Long.... NOTE: pressures on the graph are in bar, not psi....



    As you can see, increasing the preload from 2 turns out to 4 increased not only the power, but also the shot count by a couple.... going another 2 turns was a bit too much to achieve maximum shot count, but set up that way the rifle delivered 16 shots averaging 46.6 FPE at 0.97 FPE/CI, so backing it down just slightly should produce 2 clips (18 shots) at about 45 FPE with a 200 bar (2900 psi) fill and bump the efficiency to just over 1.00 FPE/CI....

    One small glitch has surfaced, something I knew I would have to watch for from previous experience with a .22 cal AT44.... The hammer spring preload tends to back itself off as you fire the gun, so it will need either some loctite or a locking device on the screw to prevent that.... I'm thinking a hole in the screw with a plastic plug inserted to create friction will probably work....

    Bob
    Last edited by rsterne; Aug 02 2012 at 08:09 PM.
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  3. #3
    Senior Member SeanMP's Avatar
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    That's what I have done a couple times for loose adjustment screws. After drilling and tpping a hole I dropped a small snip of 095 weed trimmer in the hole followed by a set screw. That gave a nice adjustable friction bushing

    Has this gun had the valve mod that you described done to it yet?
    Sean

  4. #4
    Moderator rsterne's Avatar
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    Well, today was kinda wierd.... I took the rifle all apart and matched up all the ports to smooth and maximize the flow.... I ended up with a 0.180" valve top port, 0.183" transfer port, 0.185" through the breech, and a 0.187" barrel port.... I polished the barrel with some bore paste and stuck everything back together and gained a whopping 6 fps (0.5 FPE) for my trouble.... but at least it didn't use any more air, so the efficiency was a solid 1.20 FPE/CI.... Basically all the ports downstream of the valve are as big and smooth as I can go.... By now I figure the choke point is the valve seat itself.... It looks like this....



    There was a spare supplied with the parts that came with the gun, and it measures 0.171" ID, less the area of the 0.100" stem.... so the equivalvent area is the same as a hole measuring 0.139" (the same size as a stock Disco transfer port).... This is obviously a lot smaller than the rest of my ports.... I figured if I ruined the seat I had a backup, so I knocked it out of the valve and measured it.... and found that the hole in it was even smaller than the spare, it only measured 0.163" ID.... I mounted it in the lathe, and using my number drills, one at a time, slowly took it all the way to 0.187".... the same size as the ports (but still less area because of the stem).... The throat area is now the equivalent of a 0.158" hole, 50% larger area than the one I took out.... There is enough meat to drill the hole out to about 0.200", and I can thin the stem down to 0.090", and match the throat area to the area of the rest of my ports if I need to.... However, I know that if I make the valve too big, the hammer may not open it.... so I stopped at 3/16".... stuck the gun back together and shot some more strings....

    WOW!.... What a shocker.... I still had the hammer preload at 7 turns out, hoping that it would have enough oomph to knock open the larger valve at 200 bar.... Well, the first shot over the Chrony was 990 fps, then 1010, and it peaked at 1023 fps before falling back to 983 at shot 15.... Drilling out that valve seat gained me 75 fps and 8 FPE.... and the hammer still worked !!! .... I dialed the preload down to 6 turns, and then down to 5, and the gun was still shooting way stronger than before.... I set the preload back to 7 turns out and shot a string with H&N Baracudas and EunJin Domes.... Here are the results....



    The dotted black lines on the graph are from the first string today, JSB Kings at 7 turns out, before I drilled out the valve seat.... That was the most power up until today.... The red pressure line can be used for all the pellets at 7 turns preload.... Here is a summary of the results:

    25.4 gr. JSB King: 1023 fps (59.0 FPE) max.
    1009 fps (57.5 FPE) average for 15 shots @ 1.06 FPE/CI

    30.9 gr. H&N Baracuda: 957 fps (62.9 FPE) max.
    944 fps (61.1 FPE) average for 15 shots @ 1.13 FPE/CI

    35.2 gr. EunJin Domed: 915 fps (65.5 FPE) max.
    899 fps (63.2 FPE) average for 15 shots @ 1.17 FPE/CI

    That's a 9-10 FPE gain over yesterday.... The shot count is a few short of the 18 (two full clips) that I want.... but my power goal of 50-60 FPE has certainly been achieved.... and by enough margin I am confident that I can get an 18 shot string in the 950 fps range with any pellet from 25-31 gr., and that includes all the ones with a decent BC....

    It's been quite a day.... Using JB Weld on the groove in the valve will have to wait, the tubes I have are so old they are hard as a rock.... Tuesday (after the holidays) strangely seems a long way off.... but I'll enjoy the weekend knowing I've accomplished my goal with this rifle....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  5. #5
    Senior Member sholo's Avatar
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    WOW!
    Wicked numbers your getting, Bob. That rig is turning into quite the beast indeed. I'm continually amazed at how much power can be obtained from opening things up by such small amounts.

    At those velocities in a 1/4 bore, it must be loud as a mother...!

    Todd
    Todd

    Don't sweat the petty things, and don't pet the sweatty things!

  6. #6
    Moderator rsterne's Avatar
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    Well when I bought it, I figured it would be until then before I got any time.... Fortunately, that wasn't the case.... Don't you worry about my lack of projects.... I still have a brand new (never fired) B-51 sitting here.... plus my Hayabusa, which will NEVER be finished (at least I hope not, because that will mean I ran out of ideas).... It's next version will be what Sean and I are working on, a .224 cal firing 41.5 gr. bullets high subsonic.... Then I still have .308, 9mm, .458, and .410 shotgun barrels to go on it.... See what I mean?.... Other projects that need finishing are my reversed tank versions of my 2260 and 2560 regulated HPA rifles, and I still have to fit a .25 cal barrel to my QB deluxe HPA....

    Yeah, I don't think I'll run out of projects any time soon....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  7. #7
    Member Dukemeister's Avatar
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    Bob, nice work. But you do realize winter doesn't start officially until December, don't you?
    I guess you will just have to find a different winter project......
    Member, Limestone Beekeepers Guild
    Duke ))))----//----------==

  8. #8
    Moderator rsterne's Avatar
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    Barrrel and breech correction are next on deck.... might have to drill the transfer port a whisker at the same time.... Since the valve ended up at 0.180" after I streamlined it, if there is room I'll take the transfer port and barrel out to the same size.... Epoxy between the valve screws after that (and separate, I'm curious too)....

    Pretty amazing what a bit of tweaking can do.... Like you said, nothing serious here, really just "blueprinting" and tuning....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  9. #9
    Moderator rsterne's Avatar
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    I decided to finish the job.... I drilled out the valve seat to 0.203" and thinned the stem to 0.090".... That makes the throat of the valve the same area as the valve exhaust port.... In addition, I drilled four 7/64" holes at a 20 degree angle in the inner end of the valve to allow more air to enter the valve.... This has more than doubled the inlet area, so it is now a lot easier for air to enter the valve than leave it....



    The extra holes in the inlet side should not be necessary unless the valve seat is drilled out, as the original hole was the same size as the throat, and it didn't have a stem obstructing it.... Hopefully I will find time to test again tonight.... Sorry for the slightly blurry photo....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  10. #10
    Moderator rsterne's Avatar
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    Did a bit of shooting at 50 yards in the Spring.... 25.4 gr. JSB Kings at ~960 fps from the bench....



    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

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