Maximizing power for a minimal noise level

[rsterne]

Jack, you may not have seen these charts before, but they should bring you come clarity as to what happens inside a PCP.... The first is for a stock Disco regarding what happens when the valve "self-regulates" over the normal pressure range, and automatically changes the dwell to produce a bell curve.... Note how as the pressure drops the dwell increases and the valve closes when the pellet is much further from the breech.... and also notice what happens to the residual pressure at the muzzle.... increasing as the pressure drops (causing a louder report)…. The predicted muzzle pressure roughly doubles at 1200 psi compared to 2000 psi....



The second chart shows what happens when you start by using the entire Disco reservoir volume as the plenum, and then reduce the available plenum size to 1 cc per FPE, and 1/2 cc per FPE, without changing the pressure or dwell.... The chart assumes the flow between the plenum and valve is not restricted by a small gauge port hole, so that the entire plenum volume is available to feed the valve during its dwell, which is constant for these three curves.... You can easily see the greater pressure loss when using a smaller plenum.... and the effect that has on the power....



Notice that there is approximately a 10% loss in FPE when using a 1/2 cc per FPE plenum, which would need about a 10% higher setpoint to compensate.... These charts were generated using Lloyd's spreadsheet, and while they may not give precise results, they certainly show the trends clearly....

Bob

[jackssmirkingrevenge]

Thanks Bob.  I was able to see the effects predicted by the first chart very clearly in my "out of the box" test.  It was however much less pronounced in my second test with the bored out porting, for the same consumption of air.  My interpretation is that since the muzzle energy rise is considerable, the air is expending more energy to drive the pellet and therefore there is less wasted energy expressed as noise.

In the context of what I want to achieve it seems to make sense to tune for a higher pressure and accept a lower usable shot count.

With regards to the larger plenum volume, in my current setup there is no restriction so the entire chamber is the de facto plenum.  Do you think therefore that it would make sense to accept a power loss in return for lower pressure at the muzzle by restricting the plenum volume?  My tendency would be towards keeping the large plenum but trying to reduce the valve dwell.

In the spirit of experimentation, I tried another shot string with a 2240 spring installed and a bit of preload, the results were... interesting.



I got 45% more shots for the same drop in pressure, but the velocities were all over the place.

Next step is to optimize hammer strike.  If one is seeking to limit valve dwell, what combination of parameters usually works the best?  Eg. Heaver hammer with a weaker spring/Lighter hammer with a strong spring?

[mobilehomer]

Yes, the OEM Maximus hammer spring does have a small bit of free flight.

[jackssmirkingrevenge]

Yes, the OEM Maximus hammer spring does have a small bit of free flight.

Thanks for confirming!

[jackssmirkingrevenge]

Going by this diagram from the hammer strike article it looks like a fast light hammer is what I should be aiming for:

[rsterne]

Jack, I have never had a Maximus, but I'm very familiar with the Disco.... I understood that the Maximus valve is smaller ID, because of the front receiver screws (like a PRod).... but apparently that is NOT the case, from looking at the photos of your valve.... I don't know what the ID is, of course.... The poppet is completely different than a Disco poppet.... What is the diameter of the stem at the rear, where it penetrates the valve body?.... If it is 5/32" (like a Disco), then you can replace it with an MRod poppet, which has a smaller OD and a 1/8" stem.... but your poppet may already be the same dimensions....

You second string didn't have the increase in noise because you never got past the top of the bell curve.... Most of the increase in report happens in the second half of the curve, where the velocity is declining.... and you never got there.... You need more hammer strike to get back to a proper bell-curve with the larger ports, and would pick up FPE as well.... You may have noticed something interesting with the velocity with the larger ports.... You started out at nearly the same velocity, because the shot was being limited by the very small lift at high pressures (the valve dwell very short, barely opening at all)…. As the pressure dropped and the valve opened further, you could then take advantage of the larger porting, which allowed the velocity and FPE to continue to build as the pressure dropped....

I would not decrease the plenum size, there is no advantage, and the pressure drop during the shot will require increased dwell for the same FPE....

Your experiment with the 2240 spring produced only the front half of the bell-curve, and the large shot-to-shot variation is common when the valve is operating so far from its peak (plateau) velocity on the high pressure side.... You could see that variation creeping in on the first few shots of your second string with the stock spring and larger ports.... The large ES on the high pressure side of the bell curve is caused by tiny variations in the hammer strike from shot to shot.... Since the valve is barely opening, if the hammer is travelling just slightly faster or slower, you get a big velocity change.... That gradually disappears as you approach the sweet spot (plateau) near the top of the velocity range of that particular tuning setup.... Once you are past that, and on the declining part of the bell curve, the valve is opening so far that the velocity is no longer so much dwell limited, but flow limited at the pressure and port size you have.... As the pressure drops, so does the velocity.... However, the dwell continues to increase, the closing point moves closer to the muzzle, and the residual muzzle pressure (and the report) increase....

Your second string, with the larger ports, shows you a very important thing about tuning and port size.... As you increase the port size, the peak velocity increases, but occurs at a lower pressure than before.... In addition, although you did not shoot to a low enough pressure to see it, the pressure range for a given ES (eg. 4%) decreases.... Big ports are like a hot cam in an engine.... higher maximum power, over a narrower range of pressure (/ RPM)…. The difference is, that in an engine, that power boost occurs at higher RPM.... whereas in a PCP, it occurs at a lower pressure, unless you increase the hammer strike to move the whole "power band" back to a higher pressure again.... When you do that, your entire FPE curve goes up of course, across the board....

I hope this all make sense to you.... It's a lot to absorb all in one go.... but I'll try and answer any questions you have.... I do have one for you, however.... What is your FPE goal?....

Bob

[jackssmirkingrevenge]

Jack, I have never had a Maximus, but I'm very familiar with the Disco.... I understood that the Maximus valve is smaller ID, because of the front receiver screws (like a PRod).... but apparently that is NOT the case, from looking at the photos of your valve.... I don't know what the ID is, of course.... The poppet is completely different than a Disco poppet.... What is the diameter of the stem at the rear, where it penetrates the valve body?.... If it is 5/32" (like a Disco), then you can replace it with an MRod poppet, which has a smaller OD and a 1/8" stem.... but your poppet may already be the same dimensions...

It has a 1/8" stem, there is a Hard Air article comparing the two rifles: https://hardairmagazine.com/reviews/arent-the-benjamin-maximus-and-discovery-the-same-airgun/

You second string didn't have the increase in noise because you never got past the top of the bell curve.... Most of the increase in report happens in the second half of the curve, where the velocity is declining.... and you never got there.... You need more hammer strike to get back to a proper bell-curve with the larger ports, and would pick up FPE as well.... You may have noticed something interesting with the velocity with the larger ports.... You started out at nearly the same velocity, because the shot was being limited by the very small lift at high pressures (the valve dwell very short, barely opening at all)…. As the pressure dropped and the valve opened further, you could then take advantage of the larger porting, which allowed the velocity and FPE to continue to build as the pressure dropped...

Excellent observations, your take is appreciated.  My goal would be to stay around the dwell of that small initial lift for as long as possible (say for at least 10-20 usable shots).

I would be happy with 19-21 FPE, and given that a heavier pellet stays in the barrel longer allowing the pressure at the muzzle to drop further, I would think that I should be aiming for something in the 18-25 grain range in the interest of keeping the noise down.

Your second string, with the larger ports, shows you a very important thing about tuning and port size.... As you increase the port size, the peak velocity increases, but occurs at a lower pressure than before.... In addition, although you did not shoot to a low enough pressure to see it, the pressure range for a given ES (eg. 4%) decreases....

That makes a lot of sense.

Keeping the current hammer weight and cutting on spring strength doesn't look like it's going to give me the results I want, which is why I'm considering making a lighter hammer, presuming that it will not only cut the dwell time but also flatten the curve of velocity increase as the pressure goes down.

[rsterne]

OK, so here is the rub.... When operating at the peak of the velocity curve for any given tune, the report will end up being somewhat proportional to the FPE.... Dairyboy's comment about staying under 12 FPE to be backyard friendly is probably about right....

Any PCP produces a curve something like this when you change the hammer strike at constant pressure.... This is easiest to see in a regulated gun, or one that is tethered, but it applies to unregulated PCPs if you refill to the same pressure between shots....



There is a maximum hammer strike, above which the velocity does not increase (the plateau).... Effectively, the valve is open until the pellet leaves the muzzle, so any additional air only adds to the (already loud) report and wastes air.... As you decrease the hammer strike (in this case by increasing the gap in a SSG), you reach a point where the velocity rolls over (the knee) before starting a steady decline (the downslope)…. Experience has shown that tuning for about 3-5% below the plateau velocity is the best balance between power and efficiency, but will still be accompanied by a significant (although quieter) report.... If you tune about 10% below the plateau velocity, the efficiency is increased, and the report decreased, but at some point, the gun will become sensitive to tiny variations in the hammer strike, and the ES will get really silly, as you have seen.... Going further than about 10% down the downslope is usually asking for wild velocity swings.... Also note that a heavier pellet requires more hammer strike to reach the knee (4 turns gap for a 25 gr. but 5 turns for an 18 gr. in the above)….

At the peak of the bell-curve in an unregulated PCP, you are operating somewhere on the knee of the above curve.... Here is an exaggerated drawing of what happens at various tunes in a regulated gun, which may help you to see that....



The peak of the bell curve typically means that you are somewhere on the knee AT THAT PRESSURE (the pressure where the peak is occurring, not the fill pressure).... The grey line above shows what happens if you operate on the downslope, and the further down the slope you are, the more sensitive the gun will be to inconsistent hammer strike.... However, in comparing the above curves, you have to think "backwards" if the gun is unregulated.... At the peak of the bell curve, you are on the knee.... On the front (HP) side, you are operating on the downslope, with insufficient hammer strike for the pressure (hence the wildly swinging ES if the hammer strike is variable)…. while on the back (LP) side of the bell curve, the gun is operating on the plateau, with lots of hammer strike (with stable velocity, only dependent on pressure)…. You already found that out....

When you combine how a valve self-regulates with a desire for powerful but quiet shots.... trying to do that over a wide pressure range will be virtually impossible.... If you reduce the hammer strike, to get a quieter report, you will be running well below the plateau velocity, and subject to wild swings in the ES.... and rapidly increasing velocity as the pressure drops (eg. with the 2240 spring).... The cure is to reduce the fill pressure of the gun, to get it to operate on the knee.... Then use only the fill range that will have you operating in the "sweet spot" of the bell curve, where the ES is 4% or less.... Look at your second string, and consider what would happen if you only filled to, say, 1600 psi and shot down to 1000.... Your peak velocity was 966 fps, so reduce the fill pressure until your first shot is about 927 fps, and stop shooting when it drops again to that velocity and make that your refill point....

My guess is that the gun will be much too loud for your requirements.... This is because you have increased the FPE.... Conversely, if you reduce the FPE, by reducing the pressure AND the hammer strike even further, the gun will get quieter.... Trying to get high power and low report.... without getting a wide ES.... is going to be really tough, without regulating the gun.... Digest all of this while I write my next post.... 

Bob

[rsterne]

You remember the comment I made about a larger port shortening the shot string and increasing the power?.... FT shooters, back in the days that unregulated PCPs were the norm, did the opposite.... They choked up on the transfer port and then leaned on the hammer spring to get the power back.... Here is a chart showing what happened to a Disco when I tried different port sizes (constant hammer strike)....



All the strings had a 4% ES.... You can see that the larger the port, the shorter (and more curved) the string.... and indeed the louder the report.... By the time I got down to about 600 fps (11.4 FPE) or less, the gun was quiet, IIRC.... This wasn't due to any change in dwell, however, simply less air being released by the valve through the tiny transfer port.... but it does show what happens when you change port size in an unregulated PCP....

As far as which is best, a lighter or heavier hammer, in terms of getting the least dwell for a given lift, lighter is better.... However, what I am trying to convey is that if you go too light on the hammer strike, you will end up with a string like you got with the 2240 spring, or similar.... It is possible that a very light hammer, with a bit heavier spring to keep a decent hammer energy may give better consistency of hammer strike, and hence avoid the huge shot-to-shot variation.... However, you will still end up with the velocity increasing every shot unless you reduce the fill and refill pressures to get back to the sweet spot in the bell curve, and simultaneously drop the FPE down into the "quiet" range....

Now you can see why we use regulators.... 

Bob

[rsterne]

One thing you might try which I have not done.... although I have observed the effect at times.... You could try a light hammer and relatively strong hammer spring.... and fit a stiffer valve spring.... That will require a stronger hammer strike at a given pressure, as the spring force becomes a greater percentage of the total closing force on the poppet.... This should extend the shot string by extending the range between fill pressure and refill pressure.... The light hammer with heavy spring should be more consistent (I think) in terms of hammer strike....

I haven't any real experience to know if this combination might make the gun quieter or not.... However, using large ports and short dwell is the way to get a reduced report.... It is a matter of whether you can do that and get the power level you want, with a usable ES, and a decent shot string (ie large enough pressure range)…. all at the same time....

Bob

[jackssmirkingrevenge]

OK, so here is the rub.... When operating at the peak of the velocity curve for any given tune, the report will end up being somewhat proportional to the FPE.... Dairyboy's comment about staying under 12 FPE to be backyard friendly is probably about right...

It's the extremes of the curves that are throwing me off and probably giving me a sense of false hope.

For the first string for example, the 5th shot was about equivalent to the last one in terms of velocity, but one was reasonably "backyard friendly" while the other sounded like a .22LR.

You're right of course, it's at the middle of the curve that I should be looking.

There is a maximum hammer strike, above which the velocity does not increase (the plateau).... Effectively, the valve is open until the pellet leaves the muzzle, so any additional air only adds to the (already loud) report and wastes air.... As you decrease the hammer strike (in this case by increasing the gap in a SSG), you reach a point where the velocity rolls over (the knee) before starting a steady decline (the downslope)…. Experience has shown that tuning for about 3-5% below the plateau velocity is the best balance between power and efficiency, but will still be accompanied by a significant (although quieter) report.... If you tune about 10% below the plateau velocity, the efficiency is increased, and the report decreased, but at some point, the gun will become sensitive to tiny variations in the hammer strike, and the ES will get really silly, as you have seen.... Going further than about 10% down the downslope is usually asking for wild velocity swings.... Also note that a heavier pellet requires more hammer strike to reach the knee (4 turns gap for a 25 gr. but 5 turns for an 18 gr. in the above)….

Noted.

The peak of the bell curve typically means that you are somewhere on the knee AT THAT PRESSURE (the pressure where the peak is occurring, not the fill pressure).... The grey line above shows what happens if you operate on the downslope, and the further down the slope you are, the more sensitive the gun will be to inconsistent hammer strike.... However, in comparing the above curves, you have to think "backwards" if the gun is unregulated.... At the peak of the bell curve, you are on the knee.... On the front (HP) side, you are operating on the downslope, with insufficient hammer strike for the pressure (hence the wildly swinging ES if the hammer strike is variable)…. while on the back (LP) side of the bell curve, the gun is operating on the plateau, with lots of hammer strike (with stable velocity, only dependent on pressure)…. You already found that out....

Yes!  Thank you for going over the theory again, you can look at numbers all day long but being able to relate them to real-world experience makes a difference.

When you combine how a valve self-regulates with a desire for powerful but quiet shots.... trying to do that over a wide pressure range will be virtually impossible.... If you reduce the hammer strike, to get a quieter report, you will be running well below the plateau velocity, and subject to wild swings in the ES.... and rapidly increasing velocity as the pressure drops (eg. with the 2240 spring).... The cure is to reduce the fill pressure of the gun, to get it to operate on the knee.... Then use only the fill range that will have you operating in the "sweet spot" of the bell curve, where the ES is 4% or less.... Look at your second string, and consider what would happen if you only filled to, say, 1600 psi and shot down to 1000.... Your peak velocity was 966 fps, so reduce the fill pressure until your first shot is about 927 fps, and stop shooting when it drops again to that velocity and make that your refill point....

Got it.  I think we're past the bargaining stage and heading towards acceptance, the reasonable course of action is to make an SSG and adjust accordingly, accepting the noise as a necessary price to pay for a modicum of power.

You remember the comment I made about a larger port shortening the shot string and increasing the power?.... FT shooters, back in the days that unregulated PCPs were the norm, did the opposite.... They choked up on the transfer port and then leaned on the hammer spring to get the power back.... Here is a chart showing what happened to a Disco when I tried different port sizes (constant hammer strike)....

My concern with this approach is that it would reduce power without a proportional reduction in noise, since by definition the valve will be open for longer.

As far as which is best, a lighter or heavier hammer, in terms of getting the least dwell for a given lift, lighter is better.... However, what I am trying to convey is that if you go too light on the hammer strike, you will end up with a string like you got with the 2240 spring, or similar.... It is possible that a very light hammer, with a bit heavier spring to keep a decent hammer energy may give better consistency of hammer strike, and hence avoid the huge shot-to-shot variation.... However, you will still end up with the velocity increasing every shot unless you reduce the fill and refill pressures to get back to the sweet spot in the bell curve, and simultaneously drop the FPE down into the "quiet" range....

Understood.

Now you can see why we use regulators....

Not viable for this one but definitely for my next PCP project!

One thing you might try which I have not done.... although I have observed the effect at times.... You could try a light hammer and relatively strong hammer spring.... and fit a stiffer valve spring.... That will require a stronger hammer strike at a given pressure, as the spring force becomes a greater percentage of the total closing force on the poppet.... This should extend the shot string by extending the range between fill pressure and refill pressure.... The light hammer with heavy spring should be more consistent (I think) in terms of hammer strike....

I had mentioned this earlier in the thread specifically with this in mind, in order to mitigate the effects of the reducing pressure.  I think I will give it a go before settling on a more "conventional" tune.  It's a $200 rifle I bought specifically to fiddle with anyway.

Thank you once again Bob for your considered advice, very much appreciated.

[rsterne]

Glad to have helped.... or at least not confused the situation further.... *LOL*…. Best of luck and keep us posted with the results.... It is not something normally sought after, so therefore interesting if a decent solution can be found....

Bob

[jackssmirkingrevenge]

I love the smell of Varsol in the morning!



New lightweight piston machined from aluminum compared to the original, of functionally similar size.

A 5/16-24 steel set screw serves as the impact face and I also drilled out a deeper and wider bore (1" deep, 27/64" dia.) in order to allow a great range of spring types and adjustments to be tried.

The new piston weighs 17.25g compared to the 58.16g of the original, so a little less than a third of the weight.

I also shortened and re-threaded the bolt handle, having managed to bend it during over-zealous testing when it wasn't properly tightened. 

Now to give it a whirl.

[jackssmirkingrevenge]

I found a suitable spring and chronied two shot strings, one with the spring full compressed when cocked and one with about 0.20" to spare, both shot from 2000 to 1000 psi.





That's all the time I had for testing this morning but I wonder if the answer is somewhere in between.

I'm also questioning if an SSG is worthwhile for a lightweight hammer since it's unlikely to bounce significantly due to its reduced mass.

[rsterne]

Jack, if you haven't replaced the trigger group with one from an MRod with a drop sear.... be careful with your aluminum hammer.... I am assuming the trigger is the same as the 22XX / Disco of course.... The sear can wear the hammer and cause jamming or even misfires.... The drop sear on the MRod and PRod triggers falls out of the way on firing, and will even work with an MDS (plastic) hammer....

RE the two strings with the lightweight hammer.... I would suggest that the first string, but only filling to about 1600 psi, so that the first shot is about 938 fps should give you about 14 shots within a 4% ES.... In order to utilize a 2000 psi fill, you need a stronger spring or more hammer travel.... You can increase the hammer travel by shortening the valve stem, assuming the gun works like a Disco.... Press the stem in by hand and see if it stops because the spring goes coil bound before the stem is flush with the back of the valve.... On a Disco, it sticks out about 5/16", and can only move 1/4" before getting coil bound.... Therefore shortening it 1/16" cannot affect the valve performance, and gives the hammer 1/16" more travel to gather energy and momentum.... I note that your aluminum hammer has the striker proud of the front, which actually reduces the hammer travel, not what you want when you have low velocity first shots (and hence not enough hammer strike for 2000 psi)….

If you start playing with mods like this, beware of unintended consequences.... Shortening the valve stem MAY allow the hammer pin to hit the front of the tube slot and/or the cocking slot in the receiver when fully forward, in contact with the back of the valve.... This can be hard on the pin of course.... It MAY also hit the back of the bolt on firing, again hard on the pin, and that can also cause the bolt handle to jump up on firing.... You can eliminate that by shortening the back of the bolt, but if you go too far, then it will not withdraw the hammer far enough to cock.... etc.etc.etc…. and yes, I have made all those mistakes.... 

One thing that may really help you achieve your goal is using a O-ring buffer between the valve and the hammer (2-3 of them)…. and an adjustable striker in the hammer.... This is called the "bstaley mod", and while it reduces peak power, it extends the shot string by limiting the valve lift at and after the peak of the bell curve.... This, of course, reduces the air loss and report in that part of the string.... In combination with increased port sizes (which you already have), by careful adjustment of the hammer travel, you may find a combination that gives you the FPE and shot count you want.... The O-ring buffer acts like a very stiff and progressive valve spring....

Bob

[jackssmirkingrevenge]

Jack, if you haven't replaced the trigger group with one from an MRod with a drop sear.... be careful with your aluminum hammer.... I am assuming the trigger is the same as the 22XX / Disco of course.... The sear can wear the hammer and cause jamming or even misfires.... The drop sear on the MRod and PRod triggers falls out of the way on firing, and will even work with an MDS (plastic) hammer...

If this hammer gives me the result I want, I figured I could put a steel insert later at the point where the sear engages.

I note that your aluminum hammer has the striker proud of the front, which actually reduces the hammer travel, not what you want when you have low velocity first shots (and hence not enough hammer strike for 2000 psi)….

I left it proud because the base of the set screw is slightly recessed.  If I understood correctly, I can increase hammer travel by backing the screw out a little instead of shortening the valve stem, correct?

If you start playing with mods like this, beware of unintended consequences.... Shortening the valve stem MAY allow the hammer pin to hit the front of the tube slot and/or the cocking slot in the receiver when fully forward, in contact with the back of the valve.... This can be hard on the pin of course.... It MAY also hit the back of the bolt on firing, again hard on the pin, and that can also cause the bolt handle to jump up on firing.... You can eliminate that by shortening the back of the bolt, but if you go too far, then it will not withdraw the hammer far enough to cock.... etc.etc.etc…. and yes, I have made all those mistakes....

In the old country the expression is "measure a hundred times and cut once"

One thing that may really help you achieve your goal is using a O-ring buffer between the valve and the hammer (2-3 of them)…. and an adjustable striker in the hammer.... This is called the "bstaley mod", and while it reduces peak power, it extends the shot string by limiting the valve lift at and after the peak of the bell curve.... This, of course, reduces the air loss and report in that part of the string.... In combination with increased port sizes (which you already have), by careful adjustment of the hammer travel, you may find a combination that gives you the FPE and shot count you want.... The O-ring buffer acts like a very stiff and progressive valve spring....

Do you mean like this?



Interesting.  I'll give it a go with the strike face slightly recessed, spring preload at an intermediate position between what I tested previously, then do the same with a couple of o-rings added to see what sort of a difference it makes.

[rsterne]

Exactly like that.... Try 2 O-rings, and if you can't obtain the desired effect by adjusting the striker position in the hammer, use 3 O-rings.... What you are trying to do is to allow the "normal" hammer strike and valve lift at the beginning of the string (at 2000 psi)…. but as the pressure drops, instead of the lift and dwell increasing, the hammer starts to hit the O-ring stack, at first barely touching, then more and more.... The O-rings compress more as the pressure drops, but of course won't allow the hammer to open the valve as far.... This limits the FPE, chopping the top off the bell-curve.... It flattens the curve, extending the usable pressure level.... but of course limiting the peak power.... This could be the perfect answer for your situation.... Good Luck !!!

Bob

[rsterne]

In case you are interested.... my Grouse gun is a 2260 based PCP with a 14.5" barrel, that shoots 18.1 gr. JSB Heavies at just over 700 fps (20 FPE)…. It uses a 2000 psi fill and gets about 16 shots from a tube with half the volume of a Disco.... I only use a 1/8" transfer port....



Bob

[jackssmirkingrevenge]

In case you are interested.... my Grouse gun is a 2260 based PCP with a 14.5" barrel, that shoots 18.1 gr. JSB Heavies at just over 700 fps (20 FPE)…. It uses a 2000 psi fill and gets about 16 shots from a tube with half the volume of a Disco.... I only use a 1/8" transfer port...

Familiar with this one, it's hard to research PCPs online without coming across your work how would you rate the noise?  I imagine it's significant from a shorter barrel.

Some more data in the meantime, I made a proper adjustable spring guide with a 1/4-20 screw for the lightweight hammer.

Shot string for one turn on the screw, 2000 psi to 1250 psi as that seems to be the pressure when the noise becomes more significant, still with 14.3 grain Crosman pellets:



Same setup with two turns:



Same setup with three turns:



Staying with the three turns, I added two 9/16" x 3/32" o-rings between the hammer and valve body face:



The last one definitely seems to be promising, as predicted adding the o-rings to buffer the hammer impact flattened the curve at a decent power level and with tolerable noise levels.  Thanks for sharing the concept!

I'll need to try some heavier pellets now and see how that changes the dynamic.

Incidentally, all this assembly and disassembly raises another question, what's your preferred breech screw?  Looks like it's a matter of when and not if the stock screw will strip and I understand this is a common issue.

[rsterne]

I have never stripped one of those screw's threads.... but the 0.050" hex is a PITA.... I use a Dremel cut-off wheel and slot it for a screwdriver blade.... When assembling or disassembling, press down HARD on the breech to compress the TP seal.... Then you should have no problem with it....

Your last string, with the O-rings.... it appears that the hammer is hitting the O-rings even on the first shot, as you are losing about 60 fps.... I would try running the travel adjuster in a bit (forward, towards the valve) so that the valve is opening further on the first shot, which should move the whole string up to a higher velocity, and should move the peak to a bit lower pressure.... making the string more symmetrical.... Its been a long time since I played with a bstaley mod, so I may not have that right.... After you play with it for a while, you will understand how the travel adjustment (striker position) and hammer spring tension change the string....

Try to achieve a bell curve where the peak velocity is 4% above the starting velocity, and then stop shooting when the velocity falls back to the starting velocity again.... That will give you a 4% ES, which is good to about 50 yards.... You may need to try the heavy hammer again, particularly if it is difficult to cock the gun with the light hammer because of spring preload....

Bob