Maximizing power for a minimal noise level

[subscriber]

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....

[jackssmirkingrevenge]

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

I note that there are some other commercially available options such as this one with a Torx head, 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.

[subscriber]

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. 

[jackssmirkingrevenge]

That was a polite way of suggesting I consider what numerator and denominator I use. 

It's the Canadian way

[jackssmirkingrevenge]

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:







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.

[rsterne]

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

[jackssmirkingrevenge]

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:



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, 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.

[rsterne]

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

[jackssmirkingrevenge]

I installed a much stiffer valve spring and at 4 turns the curve flattened out nicely, if at a lower energy level:



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.

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.

[rsterne]

You might also try 2 O-rings and the heavy hammer....

Bob

[jackssmirkingrevenge]

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:

[rsterne]

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....



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

[jackssmirkingrevenge]

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:



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.

[jackssmirkingrevenge]

On reflection, keeping the bolt and hammer separate is probably the best solution.

Here's the idea incorporating an stopping spring guide:

[JungleShooter]

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....

[rsterne]

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

[JungleShooter]

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