GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: jackssmirkingrevenge on November 04, 2019, 08:49:03 AM
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When I first joined this forum I thought I knew most of what there was to know about pneumatics but it was quickly apparent that I had merely scratched the surface. I've been poring over Bob's series of technical articles (https://hardairmagazine.com/author/bob-sterne/) that look at the various parameters involved and the effects of tweaking them and think I have come to the right conclusions but I defer to the more knowledgeable to check my working.
I'm looking to tune the most basic of PCPs to give the most power it can while being quiet enough for backyard plinking without disturbing the neighbors, and without the assistance of any sort of muzzle device.
In order to meet the noise requirements, it has to have the lowest possible pressure at the muzzle, so an SSG of some description is going to be necessary to open the valve only as much as is needed. I would however like to squeeze as much FPE as possible, so my reasoning would be that you want barrel pressure to jump as fast as possible at the breech end and taper off rapidly as the pellet is about to leave the muzzle.
In that regard, would it make sense to maximize airflow in terms of valves and transfer ports, then increase efficiency and reduce noise by just tapping that valve more lightly? To me it seems that in general a high flow valve that opens for a short interval is going to give better performance than a low flow valve that opens for a long interval, for the same volume of air used. I would be interested to hear some thoughts on the matter.
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Jack, That is the cool thing about these airguns: the deeper you dig, the more complex you find out they are. Or you can just shoot them and forget about it, LOL.
You know that you have asked a very complex question, correct? Muzzle pressure, caliber, plenum volume, and velocity, are a few of the factors that affect the sound. Think of how loud a popped balloon is. Maybe a couple of PSI, but lots of volume and maybe a very large caliber?? And probably too loud to be backyard friendly.
But you can easily control the muzzle pressure, and you are correct, a valve that has a quick square wave (ideal world) function would probably be best.
Here are a few internal ballistics graphs (calculated, but probably similar to real-world) to ponder. Barrel length is the horizontal axis. Pressure (blue) and velocity (green) are the right vertical axis. FPE is the red left vertical axis.
The cusp in the blue line is when the valve closes. The third chart shows the effects of a small plenum. You can also surmise how keeping the valve open overly long only wastes air and makes noise.
I hope this helps.
Lloyd
Visualize some what-ifs in your head.
(https://i226.photobucket.com/albums/dd79/loyd500/rkr-3.jpg)
(https://i226.photobucket.com/albums/dd79/loyd500/rkr-2.jpg)
(https://i226.photobucket.com/albums/dd79/loyd500/25mrod.jpg)
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Thank you Lloyd, appreciate the response.
Muzzle pressure, caliber, plenum volume, and velocity, are a few of the factors that affect the sound.
You're right, in this case I focused on pressure at the muzzle as it seems to be the most tangible parameter, as well as being a reflection of efficiency which is inversely proportional to sound levels if one understands muzzle noise to correspond to wasted energy.
Your first two charts more or less confirm what I had in my head, that higher pressure at the breech with a valve that re-seals faster has the potential to give both higher FPE and lower pressure at the muzzle.
With a small plenum it would seem that the pressure tapers off significantly faster, therefore I would surmise that one is best off with a large plenum and focus on having the hammer as tunable as possible in order to control opening time.
Is one better off with a stiffer valve spring in order to help it to re-seal faster, or will the corresponding need for a greater force to knock the valve open negate any benefit?
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Jack, you are correct that for a given FPE, using a small sip of high pressure is more efficient, and quieter, than a larger gulp of low pressure.... Basically the shorter the valve dwell, the closer to the breech the pellet is when the valve closes.... and the more room for the air to expand as the pellet heads towards the exit.... Therefore, larger ports and a shorter dwell, should be more efficient, and quieter, for a given FPE at a given pressure.... because you are releasing more HPA in a shorter time, giving it longer to expand (and producing a lower muzzle pressure)….
Smaller plenums cause the pressure to drop quicker, either reducing the FPE, or requiring a higher pressure to get the same power.... While the effect can be calculated and measured even for unregulated PCPs (compared to an infinite reservoir volume)…. it becomes very important when the plenum is less than about 1 cc per FPE.... because that is when the pressure drop during the valve open stage becomes really significant.... Here is a graph showing a gun like an MRod, and what happens to the FPE as you reduce the plenum volume....
(https://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/MRodPlenum_zps845202ec.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/MRodPlenum_zps845202ec.jpg.html)
The red dot is 1 cc per FPE....
Bob
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Thanks Bob! Good to know I'm on the right track :)
I have a used 0.22" Maximus on the way that's going to be the basis for these ideas to be fleshed out.
I understand that adding a third screw to the valve block will allow higher pressures to be safely used which would correspond to higher FPE potential.
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With the goal of having the valve open quickly for a short interval, would it make sense to go for a lighter hammer than the stock, or can the same effect be achieved with a weaker spring?
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Too generic a question.... In some guns you may need a heavy hammer and strong spring, in others you could use a light hammer and light spring.... However, lightening the hammer changes the relationship between hammer energy and momentum.... For a given spring force and hammer travel, you get the same hammer energy, but less momentum, with a light hammer.... Cracking the valve off the seat uses up energy and reduces the hammer velocity, hence also its momentum.... Lift depends on residual hammer energy, whereas dwell depends on residual momentum.... so a lighter hammer (same spring and travel) should leave the lift unchanged but reduce the dwell.... Providing you have enough lift times dwell (the area under the flow curve) to provide the FPE you want, and providing the ports are large enough to allow the required air to move in the dwell you end up with.... then a lighter hammer MAY produce more efficiency....
For a given FPE, using a lighter hammer will generally require a bit stronger hammer spring.... If you are having difficulty getting to the plateau (or at least to the FPE you want), without the gun getting hard to cock.... then try a heavier hammer.... If cocking is a breeze, then you should be able to use a lighter hammer....
Bob
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Thanks Bob, looks like there's going to be some trial and error in my immediate future. I will start with the standard weight and go from there.
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Problem with the Maximus is getting it backyard friendly without any LDC/Shroud of sorts you will likely need to be sub 12fpe. Regardless of how efficient it is it will have a pop to it with nothing.
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Bob, Those 2 paragraphs in reply # 6 are loaded with information like a water balloon that is ready to explode. As many times as we've discussed this, I still have to read it a couple of times before I shake my head "yes". Well said.
Michael, the hammer weight and spring, the return spring, the valve seat & valve stem diameters, reservoir pressure, all heavily influence the shape of the air pulse. Plus, there are about that many more influencing factors. Yes, a lot of it is trial and error, but as you can see Bob has quantified and documented that trial and error.
And each tuner probably has a few favorite winning combinations .
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Problem with the Maximus is getting it backyard friendly without any LDC/Shroud of sorts you will likely need to be sub 12fpe. Regardless of how efficient it is it will have a pop to it with nothing.
That might turn out to be the case but I'm willing to see how far I can go only by tweaking the internal ballistics.
Yes, a lot of it is trial and error, but as you can see Bob has quantified and documented that trial and error.
And each tuner probably has a few favorite winning combinations .
Yes, definitely standing on the shoulders of giants here.
The plan is to max out flow from valve to probe, as well as plenum volume, then build an adjustable SSG for tuning purposes.
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Picked up the Maximus today and was pleasantly surprised, it's no Daystate but definitely on point for the price!
I had 14.3g Crosman pellets to hand so a shot string through the chrony was in order:
(https://i.imgur.com/8LhStim.png)
What surprised me was the noise, it started as "this isn't half as bad as I was expected" and progressed to "maybe hearing protection is a good idea" about 60% of the way through. The logical explanation is that as pressure in the chamber goes down, resistance to the hammer decreases and valve dwell time increases.
That gives me a starting point, now to tear it apart and start changing things. I'll also need to get some heavier pellets.
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Jack, I can see that you are going into this with plenty of enthusiasm and are planning on learning by doing. Excellent!
A couple of things about the Maximus limit its tuneability, but I think you already know that and have some thoughts about what to do. One missing link is an adjustable hammer spring, but you already mentioned an SSG, so you have a plan there. Another issue (for some folks, anyway) is the non-adjustability of the hammer travel, i.e., no adjustable striker. The P-rod has an adjustable striker and should be a drop-in fit. Also, the t-port is fixed in size, but you can modify that as needed. The reason I mention this is because many years ago, when Crosman introduced the Marauder rifle, it was one of the first moderately priced, readily available "nice" air rifles that came with all those tuning features built-in. Plus the excellent trigger. There were enough adjustments on the gun that you could easily mess it up enough to make it shoot poorly, or unsafely.
But Crosman was working with Hans and Ray Apelles to turn the Marauder into an exceptional rifle (for the price) for the masses. They came up with a very detailed tuning procedure that could be followed by a serious M-Rod owner, to achieve whatever results they were willing to invest the time in.
There work is still valid today and is worth a thorough read for a serious tuner. Even though your goal isn't exactly like any of the many that they talk about, the theory and procedure should be very helpful, and adaptable.
Here is a link to a long article on another forum, and about half way thru the thread, somebody has posted a link to the "A-Team Tuning Guide." Take a look.
https://www.airgunnation.com/topic/tuning-a-marauder-what-to-adjust-and-where-to-start/ (https://www.airgunnation.com/topic/tuning-a-marauder-what-to-adjust-and-where-to-start/)
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Jack, you have discovered one of the curious things about unregulated PCPs, that is proof positive of the better efficiency of small sips of higher pressure.... My Discovery, the first PCP I had (and very similar to the Maximus) was exactly the same.... not too loud for the first few shot, then gradually louder and by the end of the string you needed hearing protection indoors, for sure.... especially if you shot it down to 1000 psi....
Bob
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There work is still valid today and is worth a thorough read for a serious tuner. Even though your goal isn't exactly like any of the many that they talk about, the theory and procedure should be very helpful, and adaptable.
Thank you for the resources! I appreciate that the Marauder is more tunable out of the box but since I'm reasonably handy with a lathe and mill, I elected to go for the Maximus and manufacture/modify my own parts if necessary.
My Discovery, the first PCP I had (and very similar to the Maximus) was exactly the same...
I had found this thread (https://www.airgunforum.ca/forums/topic26635.html?sid=a773502eead1b92fc848e646bdc16ac1) while doing some research, hard to believe you've only been at this for a decade. I suppose that when you're inquisitive and meticulous by nature, the result is the same no matter what you apply yourself to.
The Maximus is now in pieces and I've been studying the parts.
The first thing I noticed is that the previous owner seems to have had a go at "tuning" himself:
(https://i.imgur.com/48KcoKT.jpg?1)
Compared to this image of a standard valve, the transfer port seems to have been somewhat ineptly drilled out, and the valve stem diameter has been reduced.
(https://i.imgur.com/J6k7i7j.jpg)
My original plan was to re-make most components but after taking some measurements it seems that modifying the existing ones should do. 11/64" porting at the barrel sounds about right going by the 75% of caliber rule of thumb, and working backwards from there towards the valve the only work that needs to be done is properly drilling the transfer port and making a new transfer port sleeve. I'm thinking of making the latter in Delrin since a synthetic material should give a better seal.
I will also drill out the gauge block to 11/64", off-center to avoid weakening the component.
Is there any benefit to turning down the end of the pellet probe?
One missing link is an adjustable hammer spring, but you already mentioned an SSG, so you have a plan there. Another issue (for some folks, anyway) is the non-adjustability of the hammer travel, i.e., no adjustable striker.
Something to keep in mind, thanks. I will start with the SSG and go from there.
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Re: bolt probe. You should only need to turn the probe down if you are looking for medium to high power and the cross sectional area calculations show that the bolt probe is causing a choke point in the flow. (No reference to "choked flow" intended.)
Very good that you are handy with lathe and mill. That makes the possibilities almost limitless.
I don't know if you have bought spare parts from Crosman, but they are extremely reasonable in price (cheaper than you can make them even if you are only charging yourself $1/hr, LOL) and they charge a flat shipping fee of maybe $7 per order.
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Hello gentlemen
Despite being a novice, I've find the logic & the details comprehensible
Thanks to each of you for writing clearly.
My question is one of terminology
It seems to me an SSG is also referred to as an FFL (Hamer free flight adjuster)
Is that correct?
Thanks
Ed
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It seems to me an SSG is also referred to as an FFL (Hamer free flight adjuster)
https://hardairmagazine.com/ham-columns/conserving-air-in-pcps-hammer-bounce-and-the-ssg/
My understanding is that the SSG actively arrests the spring while a "Free Flight Hammer" uses a spring shorter than the hammer travel, what Bob refers to as the "Short Stiff Spring (SSS)" in his article.
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It seems to me an SSG is also referred to as an FFL (Hamer free flight adjuster)
https://hardairmagazine.com/ham-columns/conserving-air-in-pcps-hammer-bounce-and-the-ssg/
My understanding is that the SSG actively arrests the spring while a "Free Flight Hammer" uses a spring shorter than the hammer travel, what Bob refers to as the "Short Stiff Spring (SSS)" in his article.
Thank you, jackssmirkingrevenge
Upon rereading, and following the link provided, it appears as if you are correct
There's a lot to be said for learning by asking questions
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Some progress, made the following modifications:
- gauge block drilled out off-center to 7/32"
- valve block porting drilled out to 11/64"
- barrel porting drilled out to 11/64"
- new Delrin transfer port bushing with 11/64" ID
I then put another shot string to the chrony, with the same 14.3 grain Crosman Premier Domed Ultra Magnum pellets and going from 2000 psi to 1250 psi:
(https://i.imgur.com/Y2u9HQT.png)
I got the same number of shots for the same pressure drop, assuming the rifle's pressure gauge is accurate. Initially unimpressed by the velocity readings but as you can see, muzzle energy climbed steadily to almost 30 ft lbs.
There was an increase in perceived muzzle noise as the pressure decreased, but interestingly it was nowhere near the levels I experienced with the first string and I felt no compulsion to reach for my hearing protection.
One thing that is bothering me is the hammer. There is no pretension in the spring at all, in fact it is loose between the hammer and the rear endcap and there is about 0.20" of hammer travel between the end of the hammer touching the valve stem and the rear of the hammer touching the spring. Essentially it's a Short Stiff Spring setup. Is this normal for the Maximus out of the box? I have no memory of it being like that when I disassembled it but I'm pretty sure I put it back together correctly and no parts are missing. The rear endcap has a threaded hole but it's empty and there were no screws in there when I received it. It also feels different to cock but that might be because I did not bother attaching the stock for this second string.
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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....
(https://i378.photobucket.com/albums/oo221/rsterne/Discovery/DiscoInternalBallistics.jpg) (http://s378.photobucket.com/user/rsterne/media/Discovery/DiscoInternalBallistics.jpg.html)
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....
(https://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/Plenum%20Volume%201_zpsikpscy0e.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/Plenum%20Volume%201_zpsikpscy0e.jpg.html)
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
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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.
(https://i.imgur.com/1831EA6.png)
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?
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Yes, the OEM Maximus hammer spring does have a small bit of free flight.
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Yes, the OEM Maximus hammer spring does have a small bit of free flight.
Thanks for confirming!
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Going by this diagram from the hammer strike article (https://hardairmagazine.com/ham-columns/using-hammer-strike-to-control-pcp-power/) it looks like a fast light hammer is what I should be aiming for:
(https://i.imgur.com/O0V7XeA.jpg)
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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
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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/
(https://i.imgur.com/LcSCitZ.jpg)
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.
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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....
(https://i378.photobucket.com/albums/oo221/rsterne/QB%20on%20HPA/QB%2088gr%20Preload_zpsprl8tisi.jpg) (http://s378.photobucket.com/user/rsterne/media/QB%20on%20HPA/QB%2088gr%20Preload_zpsprl8tisi.jpg.html)
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....
(https://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/Regulator%20Tunes%20Revised_zpstv4nytay.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/Regulator%20Tunes%20Revised_zpstv4nytay.jpg.html)
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
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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)....
(https://i378.photobucket.com/albums/oo221/rsterne/Discovery/2260DiscoPortStrings2_zpsc52cf166.jpg) (http://s378.photobucket.com/user/rsterne/media/Discovery/2260DiscoPortStrings2_zpsc52cf166.jpg.html)
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
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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
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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.
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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
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I love the smell of Varsol in the morning!
(https://i.imgur.com/iEqFXlU.jpg)
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.
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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.
(https://i.imgur.com/JimslpF.png)
(https://i.imgur.com/stz83Nj.png)
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.
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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
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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?
(https://i.imgur.com/GheS3tl.png)
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.
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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
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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....
(https://i378.photobucket.com/albums/oo221/rsterne/22%20PCP/2260PCPCarbineEmail.jpg) (http://s378.photobucket.com/user/rsterne/media/22%20PCP/2260PCPCarbineEmail.jpg.html)
Bob
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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 ;D 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:
(https://i.imgur.com/Ef3Sjki.png)
Same setup with two turns:
(https://i.imgur.com/fHJ08G5.png)
Same setup with three turns:
(https://i.imgur.com/BDv0LA4.png)
Staying with the three turns, I added two 9/16" x 3/32" o-rings between the hammer and valve body face:
(https://i.imgur.com/kS1QYPI.png)
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.
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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
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Bob,
I don't know if a .25 cal barrel is available for Jack's Maximus, or what it would take to make one. However, would the 50% greater bore volume of a .25 over a .22 of the same barrel length not make it easier to achieve 20 to 30 ft.lb; along with a "low muzzle pressure"? A "quiet enough" muzzle pressure is probably one not much over 100 PSI. Yes, the wider muzzle would let out more sound, but it would be less sharp.
A .25 caliber, or a .22 with a barrel 50% longer would have about the same loudness, with the .22 being slightly quieter. Now, such a long barrel would catch on things, and whip around during the shot; so accuracy may be compromised. Hence the larger caliber suggestion...
What we don't know is the range and the quarry; if any. If a fixed range, up to 50 yards, then a .25 pellet at 600 to 700 FPS should do the job. Or is this a silly suggestion?
Dairyboy's comment about staying under 12 FPE to be backyard friendly is probably about right....
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I have never stripped one of those screw's threads.... but the 0.050" hex is a PITA....
Yes it was the hex head I was referring to.
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....
That came to mind but I thought there might be a more elegant solution. If a dremelled slot is good enough for Bob Sterne then it sure is good enough for me :D
I note that there are some other commercially available options such as this one with a Torx head (https://onlinestore.magnumairpower.com/products/custom-breech-screw), perhaps it would be a good idea to invest in a few if more Crosman-based prototyping lies on the horizon.
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....
I think I will try with only one o-ring to see what happens, I suppose it will achieve the same effect. Stacking two o-rings might also be leading to inconsistent compression as they interact with each other.
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....
Cocking effort isn't too bad on 3 turns, certainly not worse than the stock spring that is quite stiff. There is already some wear apparent on the point where the hammer contacts the sear, I will go ahead and make a steel insert for that area.
I really want to try with heavier pellets, they're obviously going to be a better choice in terms of energy vs noise abatement and what works for 14 grains might not translate well. Do you have any suggestions for this sort of barrel? I note there are some varieties in the 25 grain range, would they stabilize well or are they too much the rifling and I would be better off with something in the 18 grain range?
What we don't know is the range and the quarry; if any. If a fixed range, up to 50 yards, then a .25 pellet at 600 to 700 FPS should do the job. Or is this a silly suggestion?
There is no quarry, I like to plink but hate just punching holes in paper. If I'm shooting, I want to damage things, hence the desire to have as much energy as possible. Range would be 40 yards maximum.
I don't know if a .25 cal barrel is available for Jack's Maximus, or what it would take to make one. However, would the 50% greater bore volume of a .25 over a .22 of the same barrel length not make it easier to achieve 20 to 30 ft.lb; along with a "low muzzle pressure"? A "quiet enough" muzzle pressure is probably one not much over 100 PSI. Yes, the wider muzzle would let out more sound, but it would be less sharp.
I think the difference from 0.22 to 0.25 is more in the 30% range. I suspect that the wider muzzle would be the issue that would make up for the lower pressure. Lloyd mentioned the balloon example earlier, the human lung can only muster something like 3-4 psi but let that pressure out at once and the noise is significant.
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I think the difference from 0.22 to 0.25 is more in the 30% range.
That was a polite way of suggesting I consider what numerator and denominator I use. :)
OK; .22 is 50% larger in area compared to .177. .25 is twice the area of .177.
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That was a polite way of suggesting I consider what numerator and denominator I use. :)
It's the Canadian way :)
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Some more data, this time with only one 9/16" x 3/32" o-ring between the hammer and the valve, still with the lightweight hammer and with 1, 2 and 3 turns on the adjuster respectively:
(https://i.imgur.com/2ovfCVC.png)
(https://i.imgur.com/5pc9G7Q.png)
(https://i.imgur.com/HhOzPKR.png)
I went down to 1250 psi on the last string, the previous two were down to 1000 psi.
Focusing on the last one compared to the same pressure drop without the buffer, interestingly the extreme spread has gone down yet the energy has gone up.
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IMO, the only useful strings to compare are the last 2 strings in Reply #38 and the last string above.... All were done at 3 turns of preload, the first with no O-ring, the second with 2 O-rings, the latter with only 1.... What confuses me is that the string with no buffer didn't have the most power.... I suspect that something else has changed in the last string you shot in Reply #44, or perhaps the lower ES (and higher velocity) is from the hammer "wearing in" ?.... As you add first one O-ring, and then another, the power on the first shot should either remain unchanged (if the hammer is not contacting it), or drop.... I would have expected the first shot with 1 O-ring to be the same as with no O-rings, and for the buffer to only take effect well into the string, if at all.... Is it possible that in your last string the fill pressure was not quite 2000 psi?....
In any case, you are getting close.... The result you want is in between the strings with 1 O-ring and 2 O-rings.... You can either use a backup ring (they are half as thick) against the valve and 1 O-ring.... you can use the single O-ring and recess the striker in the hammer face.... or you can use 2 O-rings and extend the striker from the front of the hammer face.... If you move the striker, try about 0.050" change from its current position as a first attempt (1/2 an O-ring)…. Alternately, you could try increasing the spring preload to (3.5 or) 4 turns, using the single O-ring buffer and the current striker setting.... That should increase the initial velocity.... If you get the string flat enough, you may be able to shoot down to 1000 psi and gain shots.... Look for that 4% ES and ideally a symmetrical bell-curve....
Bob
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What confuses me is that the string with no buffer didn't have the most power.... I suspect that something else has changed in the last string you shot in Reply #44, or perhaps the lower ES (and higher velocity) is from the hammer "wearing in" ?.... As you add first one O-ring, and then another, the power on the first shot should either remain unchanged (if the hammer is not contacting it), or drop.... I would have expected the first shot with 1 O-ring to be the same as with no O-rings, and for the buffer to only take effect well into the string, if at all.... Is it possible that in your last string the fill pressure was not quite 2000 psi?....
I'm also surprised by this result. Filling was done with a hand pump so it's fairly easy to always fill to the same level. The only change is that I added a 1/8" steel pin to the hammer at the point where it engages the sear, but it's flush with the hammer face and I would be amazed if a hammer weight increase of 0.22 grams would make such a significant difference.
In any case, you are getting close.... The result you want is in between the strings with 1 O-ring and 2 O-rings.... You can either use a backup ring (they are half as thick) against the valve and 1 O-ring.... you can use the single O-ring and recess the striker in the hammer face.... or you can use 2 O-rings and extend the striker from the front of the hammer face.... If you move the striker, try about 0.050" change from its current position as a first attempt (1/2 an O-ring)…. Alternately, you could try increasing the spring preload to (3.5 or) 4 turns, using the single O-ring buffer and the current striker setting.... That should increase the initial velocity.... If you get the string flat enough, you may be able to shoot down to 1000 psi and gain shots.... Look for that 4% ES and ideally a symmetrical bell-curve....
The striker face is already recessed 1mm compared to the hammer face, I'm hesitant to turn it in further as I fear the pin might strike the tube. I did however give it a go at 4 turns from 2000 psi to 1250 psi, again keeping the single o-ring buffer:
(https://i.imgur.com/BJxfWar.png)
Shot count is down significantly for the same pressure drop, but the ES is practically halved compared to the result at 3 turns.
Compared to the original shot string after opening the ports, with the stock hammer and spring (https://i.imgur.com/Y2u9HQT.png), shot count is virtually identical as is the average FPE but the ES is also halved. For the same shot count and similar ES, average FPE is up around 10% compared to the original performance as I received it. Somewhat mediocre but still not bad for a first attempt, even if I say so myself.
As to my goal of having a backyard-friendly rifle, I'm not sure I'm anywhere near the result I would have liked, or if indeed it was a realistic goal to begin with. The rifle is quietest towards the beginning of the string, but inevitably power and noise increase considerably as you go further along. It seems to be that the ideal would be as flat a curve as possible in the beginning and accepting a lower usable shot count. I'm wondering if the best would be to take it to higher pressures in order to raise the peak performance. I notice there's a third hole at the bottom of the tube in line with the valve body retaining screws, would adding another screw here suffice to be able to take it to say 2500-3000 psi?
The only other variable I haven't touched yet is the valve spring, I think this will be my next immediate step.
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IMO from that last string (which is pretty good), you want the O-ring buffer to engage a bit sooner to reduce the peak power slightly and flatten the string more.... To do that, you need to either recess the striker further into the face of the hammer, or add a thin space between the O-ring and the valve.... The former MAY increase the velocity of the first shot somewhat, or it may not.... Adding the spacer MAY reduce the velocity of the first shot, but more so the rest of the string.... If you add a spacer, you may in addition need to change the position of the striker.... You are very close, it will only take some small amount of tweaking from where you are to get to that 4% ES....
I am not familiar enough with the Maximus to know about the lower valve and tube hole.... but it seems that may be a possibility.... HOWEVER, if anything, with the curve you have currently, increasing the pressure is the last thing you want to do, IMO....
Bob
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I installed a much stiffer valve spring and at 4 turns the curve flattened out nicely, if at a lower energy level:
(https://i.imgur.com/v9HViqJ.png)
Once again though, the noise levels towards the end were high enough that I reached for my hearing protection.
Dialed it down to 3 turns and although I only shot till 1500 psi this time, it looked like the curve was going to settle at only a little less energy.
(https://i.imgur.com/WwDHmXb.png)
add a thin spacer between the O-ring and the valve
I'll try that, accepting that if I want lower noise, I'm just going to need to dial it down a couple of turns.
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You might also try 2 O-rings and the heavy hammer....
Bob
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I've put around a thousand pellets through the chrono and the cocking system is starting to wear my fingers out, especially at greater spring compression. The fact that the bolt pushes the hammer at an angle tends to make the latter bind in the tube. Time to rethink it a little, maybe have a separate hammer cocking and pellet loading in the style of early pump rifles:
(https://i.imgur.com/DXAlsFQ.jpg?1)
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Make sure you have a large enough knob of a shape that it doesn't slip from your grasp and fire the gun.... Here is what I have used....
(https://i378.photobucket.com/albums/oo221/rsterne/30%20cal%20Disco%20Double/IMG_3436_zpsd547d809.jpg) (http://s378.photobucket.com/user/rsterne/media/30%20cal%20Disco%20Double/IMG_3436_zpsd547d809.jpg.html)
The spring guide and cocking handle add significant mass to the hammer, of course.... Using the stock, heavy hammer will lighten the cocking force required significantly compared to your aluminum one.... worth a try with 2 O-rings....
Bob
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Make sure you have a large enough knob of a shape that it doesn't slip from your grasp and fire the gun.... Here is what I have used....
Nice!
I wonder if one could still integrate the two, of course you'd need a locking mechanism of some sort to prevent the probe from being opened by the pressure but that would be easy to figure out. Here's the rough concept I have in my head, approximately to scale:
(https://i.imgur.com/jci1srF.gif)
It would be a little difficult to have an adjustable hammer spring preload though, and the probe would have to be hollow to accommodate the rear breech screw. From a mechanical point of view it would be nice to have a single movement though, I'll see if I can come up with something viable.
The spring guide and cocking handle add significant mass to the hammer, of course.... Using the stock, heavy hammer will lighten the cocking force required significantly compared to your aluminum one.... worth a try with 2 O-rings...
I wanted to try that but the cocking situation was getting progressively worse even with the stock spring, sometimes the hammer would bind and would be impossible to move enough for the sear to catch it so testing was getting a bit frustrating, hence the desire to remedy the situation.
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On reflection, keeping the bolt and hammer separate is probably the best solution.
Here's the idea incorporating an stopping spring guide:
(https://i.imgur.com/jWh3I2y.gif)
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Jack, you have discovered one of the curious things about unregulated PCPs, that is proof positive of the better efficiency of small sips of higher pressure....
My Discovery [...] was not too loud for the first few shot, then gradually louder and by the end of the string you needed hearing protection indoors, for sure.
A related question — what about the noise level of a regulated PCP?
For a given muzzle energy — would there be a significant difference in noise in one of the two following configurations?
Config 1
Plenum: large
Regulator set point: low
Config 2
Plenum: small
Regulator set point: high
Thanks! 👍🏼
Matthias
PS: Yes, I've been trying to justify buying an extended plenum, but this post has sown some doubt....
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Significant difference?.... Probably not.... The biggest governing factors are the pressure, barrel length, port size and FPE produced.... Assuming you are not running ths regulated PCP "wide open" and therefore an air hog.... there are only a narrow range of solutions to the "constant FPE, porting and barrel length" equation.... If you have too small a plenum, you need to increase the pressure to get the required FPE.... The pellet will get a slightly harder kick initially (from the higher pressure), but the pressure will drop more quickly during the shot.... Increasing the plenum size will allow you to reduce the setpoint pressure, but maintain that pressure better during the shot.... If in both cases you are closing the valve when the pellet is about 1/4 of the way down the barrel, there won't be a significant difference in muzzle pressure and hence report.... Most changes in muzzle pressure come from where along the barrel the valve closes, and that also pretty much sets the FPE.... So, there might be a slight difference in report.... Enough to hear?.... I doubt it....
Bob
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Thank you, Bob, for removing one of the excuses that the more rational "me" was trying to bring up against buying that extended plenum. ;)
Matthias