Need a little advice tuning a regulated gun

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UCChris

UCChris

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Mar 12, 2017
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Chicago, IL, United States
Hey all, need a little advice since it's been a few years since I've tuned air rifles. I'm working on my wife's Prod since it had a ton of hammer bounce. This gun has an in tube Huma regulator and factory weight hammer. My problem is that I'm getting a larger ES and SD than I think I should be. The gun has a 20" Mrod barrel and is shooting around 13-14 fpe, I'm unsure of the regulator setpoint.

Anyway, I went to a shorter stiff HS to remove the hammer bounce. The average velocity has stayed the same, so that's a net positive. The ES over the string is 30 fps. If I recall, I think the regulator might be set to the highest pressure.

IIRC, one of the potential reasons for a larger than expected ES on a regulated gun is a higher reg setpoint imbalanced with a too soft hammer strike. I'm going to pull the reg and drop the setpoint since I don't want to increase the cocking force for my poor wife's arthritis. But I wanted to check and see if I'm on the right track.
 
Keep increasing hammer spring tension* until the velocity no longer increases. Then back it off until the velocity falls to 95 - 97% of the maximum...what we call the velocity knee. At that state of tune, the gun will typically have the lowest extreme spread when taking into account inevitable variations in hammer strike, regulator output pressure, etc.

If you can't get enough hammer strike to make the velocity plateau (i.e. hitting the valve harder causes no further velocity increase), reduce the regulator's setpoint and go back to the start.

* or for guns that have an adjustable stroke (Marauder, et. al. whose hammers have a threaded nose), you can also try increasing the stroke. Either/both have the effect of a harder hammer strike.
 
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Vetmx said:
You guys keep talking fpe like we live in Europe. What weight pellet are you shooting and how fast. Actual pellet weight can have a bearing on your numbers and what’s going on inside your gun. You can have a horrible ES with an 8.4 and a great ES with a 10.3 all at the same vague fpe.
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JSB 10.44 @ 777 fps avg

nervoustrig said:
Keep increasing hammer spring tension* until the velocity no longer increases. Then back it off until the velocity falls to 95 - 97% of the maximum...what we call the velocity knee. At that state of tune, the gun will typically have the lowest extreme spread when taking into account inevitable variations in hammer strike, regulator output pressure, etc.

If you can't get enough hammer strike to make the velocity plateau (i.e. hitting the valve harder causes no further velocity increase), reduce the regulator's setpoint and go back to the start.

* or for guns that have an adjustable stroke (Marauder, et. al. whose hammers have a threaded nose), you can also try increasing the stroke. Either/both have the effect of a harder hammer strike.
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Thank you Jason. I was able to get up to 870-ish with a full length heavy HS, which I then trimmed back until the hammer bounce stopped and now we're at 777 avg. So it would make sense that I'm way below the knee. I'll drop the reg pressure and go from there.
 
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UCChris said:
I was able to get up to 870-ish with a full length heavy HS, which I then trimmed back until the hammer bounce stopped and now we're at 777 avg. So it would make sense that I'm way below the knee.
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Agreed, that represents over a 10% reduction from the plateau instead of the typical 3 to 5%.

UCChris said:
I'll drop the reg pressure and go from there.
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Keeping my fingers crossed the bounce doesn't return! Lowering the pressure makes it more susceptible to bounce.
 
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You get more dwell as long as you have a spring constantly leaning on your hammer. Once you achieved free flight your dwell changed and things got a little whacky. Right now you need to send that hammer faster with the same length spring. So you have to either get a stiffer spring or up the hammer weight with the spring you cut. I would just get a heavier spring. It won’t take much.
 
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The short stiff spring is an effective path to take to avoid the hammer bounce blues. The drawback is a SSS that actually gives you the velocity that your bouncer did is it’s a bastard to cock. Unless you have a gun with a cocking lever. It’s basically the same with a SSG. I recently sold a Prod with a SSG and told the guy that it’s tuned like a $1,500 gun but your kids aren’t cocking it
 
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What really worked well for me when I regulated my 1720t.

Stock spring, maybe a slightly stiffener one? Was to do the opposite of what we would normally do with free flight to reduce hammer bounce.

I ran the striker almost all the way out and hit the valve with a very short quick jab. This knocked down hammer bounce considerably. It went from a blubbering shot cycle to a nice quick pop.

Takes a little trial and error to hit your target velocity. But you'll definitely hear the shot cycle tighten up when you get there.
 
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So, theoretically you should have much better es. But I recon your valve might be sticking. Or something in the hammer tube or hammer it's self may be causing some friction. Something is not operating consistently. It could be the reg as well. I'd try a lower pressure setting on the reg and see if the valve or reg isn't any happier. Of course you'll have to do hammer adjustments too.
 
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Long_Gun_Dallas said:
So, theoretically you should have much better es. But I recon your valve might be sticking. Or something in the hammer tube or hammer it's self may be causing some friction. Something is not operating consistently. It could be the reg as well. I'd try a lower pressure setting on the reg and see if the valve or reg isn't any happier. Of course you'll have to do hammer adjustments too.
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Not necessarily. He has a 20” barrel in .177 and is pushing a 10.3. His valve may need to stay open just a little longer but shorty just isn’t sending the hammer fast enough to get the valve pin compression he needs. It’s a fine line with these guns.
 
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I’d try a weaker the valve spring like a Hill #7 and a light hammer. Its also a massive tp port (.18”?) maybe slim that down to be more consistent with the barrel and valve ports. Also check the metering thingy screw on the reg to make sure its filling fast, can even grind a flat spot for faster response.
 
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Well, I had a very eventful evening turning this regulator down. For some unknown reason, I decided to not put a string on the filter screw, so I had to disassemble the gun to get to reg out. Not a problem, right? Prods just have a few screws, right? Nope! This Prod is in a Rapid70 bullpup stock. So I had to take the entire thing apart.

Got the regulator out finally, it was set to the 125 bar that it ships from Huma set at. I turned it down to 105 bar, tied some monofilament fishing line to the filter screw, and screwed the filter screw back into the reg. This is where I made my fatal error. I was distracted while screwing in the filter screw and failed to realize that the knot from the monofilament line had slipped into the slot for the pressure adjustment and I was screwing the adjuster in. For anyone unfamiliar with a Huma reg, the pressure adjustment range is made in slightly less than a single 360° rotation. I had rotated the adjuster an unknown number of rotations at this point, and didn't know the height of the adjuster at the correct revolution.

At this point I was over an hour into a regulator adjustment that should have taken less than five minutes. After a quick smoke break to compose myself, I formulated a plan. I turned the adjuster to where I thought I was at least a revolution high and set the hash mark to 105 bar. I then grabbed a Prod gauge block/gauge from the parts bin and installed the regulator with the gauge so I could see the output pressure. 105 bar is 1595 psi, so I figured I would slowly fill the gun to 2000 psi and see if the plenum gauge stopped around 1595 or not. It did not, so I degassed, pulled the regulator, spun it in 360° and set the hash mark back to 105 bar. After a few cycles, the regulator showed that it was regulating again. Now, after 2 hours, the regulator was adjusted. I then pulled the tube apart again to remove the gauge and block, and reinstalled the reg. Then I had to reassemble the whole bullpup kit again. I have it pressurized now and doing an overnight leak test.

TLDR - Life sucks sometimes, the reg is adjusted to 105 bar, I'll chrony it tomorrow.
 
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Wow, sorry you had to go through all that busy work but I also had to laugh because it reminded me of this story from a similar "oh, this won't take long" project from a couple of years ago...

nervoustrig said:
Last night I had a fun little experience that I just wanted to share. Earlier in this thread I made a comment about the new PEEK poppet and that I was happy it sealed from zero pressure straight away. Well, I made one for a different QB this weekend and went to reassemble things last night, and it ended up being one of those "shoulda just gone to bed" experiences. Everything went together nicely and I heard no hissing as I began to pump it up. I put in 3 pumping sessions and had it up to ~1500psi and then stopped for a few minutes to put the little ones to bed. When I returned, there was a lovely soft pssssssshhh greeting me. I soon realized it was coming from the Ninja regulator bonnet. I think "Oh great, I get to start over again."

So I proceed to let my hard work return to the ether and remove the bottle and open up the regulator. Once inside, I see the -008 polyurethane O-ring at the base of the spool looks like chewed bubble gum. So I remove it and then notice the Belleville washers have scored the spool. Off to the shop to deburr them and clean up the spool. Cleaned the parts and reassembled and decided I had better check the setpoint. To do that, I need to remove the regulator and thread it into a tiny tank so I can bring it up to pressure with just a few strokes. Oh yeah right, this reg has never been removed from the bottle so back to the shop to clamp it in the vise and use the strap wrenches to separate them. Back in the house I check the pressure and it's 1300psi instead of the intended 1500. Back into it to add a .005" shim and that brings it up to 1500.

Now I return to what I was originally doing. Put the bottle back on and start pumping again. What's this? A hissing sound from the poppet now? I have no explanation but it was sealing before so I decide to remove the bottle and bring it up to 500psi or so and then thread it back on to help seat the poppet. I start pumping and the bottle is steady dumping the air past the check valve. You gotta bed kidding me. I disassemble it again and replace the seal and get back to pumping and bring the bottle up to 500psi. I thread it back onto the tank block and the poppet is still hissing, very faintly though so I decide to keep pumping and see if it seals with more pressure on it. At 1000psi it's still hissing. I'm this far in so I might as well keep going. I take it up to the 1500psi setpoint and it's still hissing.

Ugh. Dump the pressure again. Remove everything, pull the valve and head back out to the shop to lap the poppet again. A few minutes spinning it against the flattening jig (simply a hardwood block with a hole drilled in it), refreshing the sandpaper often, and then in the valve itself for another few minutes with a slurry of J-B & polishing compound. Back inside to wash and dry the parts and begin reassembly.

I start pumping and finally the angry air pixies are staying where I want them.

Is there a moral to this story? Yes, don't start a 1 hour project at 10pm because those projects get done at 2am.
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Pardon my busting in on your thread, but I spent most of New Years Eve wrestling with a Huma P-Rod reg. I too got off one revoloution on the reg adjustment as the filter screw was tight, and removeing it threw everything out of wack. It is very touchy, you have to go 10 bar past your desired set point then back it down 10 bar. Weird. Finally have it putting out 2000 psi according to the still installed gauge. I made a tool out of a coat hanger to reach the screw on the reg for easy removal without complete tear down.
 
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nervoustrig said:
Wow, sorry you had to go through all that busy work but I also had to laugh because it reminded me of this story from a similar "oh, this won't take long" project from a couple of years ago...
Click to expand...

Oh man, that gave me a chuckle. Angry air pixies 🤣. On a more somber note, last night did remind me that I shouldn't be working on HPA guns when distracted. At most it would have cost me a $100 regulator and some time, but it could be something more serious next time.

In good news, dropping to 105 bar netted +2 avg FPS, -10 fps ES, and no discernable hammer bounce.

Screenshot_20230102-104944.png


I keep getting lucky with these Prod projects!!!
 
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