Stoeger Stoeger XM1 Bullpup How Tune Down?

[roaoro]

Recently acquired the Bulllshark in .177. Actually shoots quite well for it's price. It currently shoots around 1100 FPS w/ crossman premier HP too hot for my needs. I'd love to be able to get this thing down to under 800 FPS. Is that possible and how? I can't seem to find any tuning instructions on this specific rifle. I'm assuming this can be adjusted via hammer spring tension and or regulator adjustments, but have no idea where to start. Any help appreciated.

 

[Florida_Man]

I have done regulator adjustments from both ends but lately I've just been doing it from the back like Florida_Man. Doing it from the fill port end of the airtube doesn't work if the regulator has been out recently or if the lock nut is pretty tight. You are depending on the O-rings to be stuck enough for you to get the lock nut loose. That would not appear to be what happened to roaoro but is it possible you did not get the lock nut loose so you just rotated the regulator with the screwdriver (I've done it)? In any event doing it from the rear on a Bullshark is not clearly a lot more work, depends on how good you are with a 18 inch screwdriver. After degassing the gun, I just loosen the two 3mm allen head screws on the clamping mechanism and unscrew the airtube from the action. Then take off the back nut on the airtube and if you are lucky the regulator comes with it. If not, you have to take off the nut on the other end of the airtube and use your 18 inch extension to push the regulator out. Taking the airtube off does make it necessary to get the clamping mechanism reclamped in the same spot but leaving the action in the stock (with the stock nuts loosened a bit) can help them not shift. Another helpful step for getting the clamping mechanism right is to have the safety on. That limits how far the mechanism can slide. I find having there be a little clearance between the safety button and the trigger guard is about right. I check to be sure it will cock, release the sear, and not release the sear with the safety on. There are positions where any of these will not be true.

Half a turn on the adjustment screw is a lot. I've done it but I've gotten my regulator messed up doing it too. I would try 1/4 turn. If you google up a picture of the Huma you can see the marks on the paper tape are a fraction of a turn apart. The SPA regulator is not identical but it's similar. It doesn't take much of a turn to make a meaningful change.

An advantage of taking the regulator out is you can more carefully control your adjustments. I've started measuring the total length of the regulator with my digital calipers. Lengths around 50.5mm seem to work. I measured the regulator of my P35-22 at 50.57 inches and then shot it down. I think it came off reg about 140 bar but it is a little unclear because the velocity started to modestly climb, not go down. I think the hammer spring might be a tad light for the regulator. It is not a terrible way to have it, however, since I was still getting very usable shots at 130 bar but it was going down at that point. The velocity peaked about 15fps faster than it had been. A full turn of the regulator screw will change it's length by 0.5mm.

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Great information, thanks for sharing. I’ve been busy with family stuff this weekend so I’ve only popped in.

 

[JimD]

I finally got my P35X shooting, sort of. It was a present from the former owner, I think because he was frustrated with it. It has been a struggle to get shooting after I messed it up some trying to get the power level up. It is thus an unusual gun. But I deliberately set the regulator low because it struggles to open the valve. I set it at about 50.4mm length. The regulator appears to be set at 90 bar.

The useful range for the SPA regulator appears to be from about 50.4mm to about 50.6mm. This is probably a bit wrong due to manufacturing tolerance but it is a range to try. That is less than half a turn of the adjustment screw.

 

[roaoro]

I finally got my P35X shooting, sort of. It was a present from the former owner, I think because he was frustrated with it. It has been a struggle to get shooting after I messed it up some trying to get the power level up. It is thus an unusual gun. But I deliberately set the regulator low because it struggles to open the valve. I set it at about 50.4mm length. The regulator appears to be set at 90 bar.

The useful range for the SPA regulator appears to be from about 50.4mm to about 50.6mm. This is probably a bit wrong due to manufacturing tolerance but it is a range to try. That is less than half a turn of the adjustment screw.

That sounds like a useful tip for length setting. As I am trying to get my FPS down to around 800-850, do you think 50.4mm would be a good starting point? Not sure where it is at since trying to adjust it from inside the tube previously. I'm about to attempt to take it out from the front as suggested. Definitely not a fan of taking it apart, so hoping I can get it close this time.

 

[roaoro]

Ok got the regulator out and measured where it currently is at. 54.1mm which is after I maid the initial 1/2 turn CW through the tube. So I am assuming that I did adjust it correctly the first time. @JimD when you say the useful range is from 50.4-50.6mm, is that basically the range between the 2 o-rings? I'm trying to understand how this works. I'm assuming the outlet hole needs to stay between the 2 o-rings, and by adjusting the adjuster screw clockwise until the lower o-ring (in pic) is almost touching the outlet hole, this will get me the lowest setting for the regulator?

IF that is correct, I'm thinking I could get a more accurate measurement with the adjuster out of the housing to have a better idea as to how many turns I need to make to get the lowest setting. Or do I have this all wrong???

 

[JimD]

There is a picture of how I do the measurement in post 18 of this thread. It is not based upon a theory of how regulators work, it was just a guess initially. Then I tried it a couple times, made measurements and then shot the gun down to below the regulator setting. Experimentally instead of theoretically determined, in other words. The range of lengths I suggested is based upon the length of the regulator in my P35X right now which seems to be giving me about 90 bar and the length of the regulator in my P35-22 which seems to be about as high as the hammer spring will open (140 or so).

What I don't know is how much variation there is between SPA regulators. I wouldn't be surprised with a 0.1mm variation. But that is purely a guess.