Taming the bark of an Avenger

[subscriber]

Peter (soupyyy),

Your redesigned air stripper with multiple reflex paths to the rear of the shroud looks like it should be very effective.

My comment that you reacted to had a second component:  People argue about the "best" distance to place the air stripper wall (or cone lip) from the barrel muzzle.   I am attempting to satisfy all the objections/principles by means of a multi-stage air stripper design: 

Certainly, the closer the barrel muzzle to the first wall, the less the pellet is buffeted by the initial "uncocking" blast that occurs as the pellet just clears the muzzle.  But, the closer that wall or lip is to the muzzle, the more air follows the pellet through the baffle bore, because the air has had less distance to travel and "fan out".  This makes a "close" airstripper better for grouping, but louder, because it is less able to direct air to the rear of the shroud (so the theory goes).

So, my proposal was to create a multi-stage air stripper - see attached images.  The design I just did to try and illustrate the point has the first "wall" one caliber (.25") from the barrel muzzle.  There are two symmetrical air channels on either side, flowing to the rear of the shroud.

Then, measured from the back of the first air stripper wall is a second wall, with its front face two calibers away (0.50").  Also having two channels running towards the rear of the shroud; next to the first two channels.

Then, in this design, there is a third wall spaced three calibers away from the rear of the previous wall (0.75").  With its own pair of rearward channels.

From then on, what you see is a conventional shroud extension with a number of flat baffles (could have been conical).

If you would like the STEP file of this design to take a closer look, PM me.

[subscriber]

Clarification:

The above design has a 7 mm bore; intended as a tight .22.  The air stripper baffles are spaced at .25, .5 and .75" as stated.  So, more like the bearing length of a .22 pellet; then 2 and 3X that distance.  I got myself caliber confused because it is either very early, or very late - depending on how you want to look at it

If I was going to do such a design for myself, I probably would make it with only two air stripper baffles.  Three may be too many, but even that can do no harm.

The section that would fit into the shroud is 1" in diameter in this design, because I don't know the actual dimension.  And this is just to illustrate a deign in principle.

[JimD]

I measured the inner diameter of my Avenger shroud at 22.5 mm last night after posting.  I then ordered some 20mm hair curlers from Amazon.  I had to buy a couple dozen but it was still not much money.  I think with a layer or two of felt these should work fine.  I will try a simple 3/8 washer separating the hair curler (probably half or one third curler) sections but I think it may be too big an inner hole and too small an outside diameter to dampen sound well.  If so, I plan to grind down some 5/16 fender washers to be a slightly loose fit in the shroud.  Low tech but also low cost.  We'll see what it does.  I'm thinking I will remove the DonnyFL adapter and just add these pieces inside the shroud after the centering device.  After I do that, unless it gets real quiet I will add some small  holes at the back of the shroud.  That is the only step where I am permanently modifying the gun so I will do it last.  I will cut the curlers to allow at least 3 curler/baffle combinations in the shroud, I will do 4 if the resulting curler length doesn't seem too short.  I will have enough curlers I can easily do both. 

I might also try leaving the DonnyFL on the gun, I will definitely do it if I get clipping.  I did not have clipping before I put the DonnyFL on so I am optomistic I can get by without it.  But to use it, I would add more holes to let the air out sooner and the baffles will need a bigger inner hole.  The advantage is a better supported barrel, the down side is it takes up space in the baffle which will reduce the effectiveness, I think. 

[soupyyy]

Subscriber, thanks for those very detailed images.  I am a visual guy so that really helped a lot.  I am on it and will post some pictures and number tomorrow.

[Back_Roads]

The air pressure in front of the pellet depends on the density of the air, the length of the barrel and the average acceleration of the pellet.  As air is not very dense, the air in front of the pellet is much less significant than the air behind the pellet.  The air pressure behind the pellet depends on the supply pressure, plenum and barrel volume, valve duration and transfer port area and efficiency. 

It is the average pressure behind the pellet that predicts its energy and velocity.   But when considering report, it is muzzle pressure and volume that matter.  Muzzle pressure is a much lower value than average air pressure down the barrel; unless a "dump valve" is used.

That said, evacuating the air ahead of the pellet should enable it to reach a higher muzzle velocity.  Here is a "PCP" shooting a baseball at over the speed of sound, with the help of a vacuum pump to remove most of the air in front of the ball (while in the barrel):

 I'm sorry but here was the next in line on You Tube

[soupyyy]

I got some number this morning before work.  The numbers are too close together to have a conclusive answer as to which layout is better.  It is interesting though.

[subscriber]

I got some number this morning before work.  The numbers are too close together to have a conclusive answer as to which layout is better.  It is interesting though.

Man, you are fast; Peter.  At least the three stage air stripper version is not louder than the single stage one .  Now the question is, does it tighten groups?  We know that it increases the total length of your plug-in device.  And that may be less desirable.

I suspect there is a law of diminishing returns.  Eventually the long path from the front air stripper adds as much flow resistance as simply having fewer air strippers with a shorter path. 

In theory, splitting the air stripper should reduce pellet buffeting and tighten groups, over a single air stripper configured for maximum air volume diversion.  Even if it does not make it much quieter.   For a solution to be better, the previous solution needs to be considered lacking.  Else, at least qualitatively, we have a solution to a nonexistent problem...

As always, the devil is in the details:  The name of the game is reduced air flow resistance.  Hence maximizing flow area, with the shortest flow path; rather than just the number of "feeder troughs".  I have attached the air stripper design I showed earlier, but with an overlay in blue, of a trough design optimized for flow area. 

I think that two airstripper walls, with a full width pair of troughs each, would optimize the flow area and length equation better (because there is less total side-wall to eat up air flow area, and an overall shorter path length).   Now the question becomes, at what stand-off distances, each?  One and a half calibers, and three calibers, perhaps...

What stand-off distance did you use for your single stage air stripper; and for each stage of your three-stage design?

Now, making the side troughs deeper does make the wall with the central bore hole, narrower.  If it is too narrow, that wall stops acting like a "wall", and become a tube.  The walls serve an important function:  Air flow stalls against the wall and is converted into pressure.  That pressure then feeds the exit path.  If there were more room, one could curve the path to help redirect the flow backwards, rather than stall it. 

So, it is possible to optimize one aspect of a design at the expense of another.  Exactly when the design "breaks" is a matter of testing.  Looking at existing designs out there helps with intuition.  I don't have flow design software, other than attempting to balance several factors in my head.  Veritable fuzzy logic

The air strippers made by/for Hatsan directly below have the ability to adjust the cone "stand-off" distance from the barrel muzzle.  Yes, these vent to ambient air, purely to tighten groups, but my point is that the stand-off distance is not simply so many inches, or so many calibers.  It is a matter of experimentation (or an educated guess).  It depends on many factors, including the tune; that affects muzzle pressure and volume of air that needs to be handled.

https://www.pyramydair.com/product/hatsan-air-stripper-for-select-22-cal-hatsan-air-guns?a=4557

[subscriber]

Peter,

The reason I modelled flat air strippers is because I don't know the capability of your printing (I assume the large diameter end is "down" on your platen.  Or you are printing with the device laying on its side). 

If possible, I would make the front face of the air stripper at least slightly conical; or with a lip to encourage air to spill sideways, rather than also towards the central bore.

One does not need to make a very deep cone to help.  Deep cones are longer and increase the overall length of your plug-in.

You probably should make the exit window cuts that join the air stripper wall to the ducts, larger.  Those vents look like choke points in your design.  Otherwise, your parts look jolly nicely made.

[soupyyy]

A GTA member shared with me his suppressor design and I printed it and ran a test and it did better than my design by 1.2 dB at only 77.8 dB.  On top of that his suppressor is 2.5" shorter.  Shooting the Liberty .22, 2900 PSI, 756 fpe.  I think the slanted wall and larger opening for air flow really worked better.  His design is similar to the one in the picture.

[subscriber]

Those slantly baffled cans work, or else none would exist.  My concern is asymmetrical turbulence, buffeting relatively light weight pellets about.  That said, if groups don't open or shift in an unacceptable manner, then the buffeting problem is not showing itself in practice.

[Lobo]

Thanks Lobo,

I was thinking generically of the Marauder and FX type setup.  Hence my request for a picture.  Should have found an actual resource, such as this video: 

  (starts at the point the shroud is removed).   

It shows the barrel to shroud support as something that could be moved from its factory position.

No problem! It is simply timing that left me able to answer that question. I am brand new to PCP's, the Avenger is my first one. The first rifle that was sent to me had issues, and in trying to figure out what was causing the issue I removed the shroud.

Lobo

[JimD]

I had mine apart today messing with hair curler mods (no success so far).  The barrel to shroud spacer is just held in place by a grub screw so it can be moved.  But there is a divot in the barrel for the stock location.  It's a couple inches down the barrel, not right at the end. 

[avator]

Everytime I hear about you guys adding this and adding that to quiet your guns I am reminded how fortunate I am to live where I do. The Avenger is among one of the quietest guns I own.
Thank y'all for that.

[JimD]

Nobody has complained about my Avenger but I don't want them to start either.  Over 100 db just seems loud for a residential area to me. 

I gave up on making it quieter with hair curlers and home made baffles.  My baffles did not fit the shroud well which may have been the issue.  I could buy a kit from Hajimoto for about $50 and try deleting some of his baffles to put in some hair curlers but I decided instead to just buy a DonnyFL Tanto.  Should be here in a couple days.  I never got a significant reduction with my changes other than the home made moderator that started clipping. 

[JimD]

I got my Tanto a few minutes ago and immediately set up and gave it a quick test.  My average sound measurement dropped from 100 db to 86.6.  I am pretty happy with that.  It makes my 48 fpe Avenger almost as quiet as my 17 fpe Prod.  Not nearly as quiet as my 32 fpe P35 but I think mid 80s is backyard friendly enough. 

Not sure about other effects, I will try and shoot it more tomorrow.  I shot a group while taking sound measurements but it was a little bigger than I'm used to for that rifle.  It was probably me.  I want to check velocity too.  Now that it is reasonably quiet I am more comfortable shooting it. 

If you decide to put a moderator on yours you definitely want one about the diameter of the Tanto - not bigger.  Even then you need the DonnyFL adapter to push the moderator out a little so you can get the fill cap off. 

[JimD]

I shot 6 three shot groups with the Tanto on my 25 Avenger.  I was measuring velocity but not sound during this shooting.  I am now confident that the Tanto does not affect the accuracy of my Avenger - positively or negatively.  I only shot one group at 25 yards and it was about 1/4 inch, maybe a touch smaller.  The other 5 were at 33 yards and were 5/16, 5/16, 1/2, 9/16, and 9/16.  I was not concentrating as well on the bigger groups.  I find I need to get the gun supported to where I don't have to push on it at all to get the crosshairs where I want them if I am going to shoot the smallest groups.  I shoot better when I move the supports, not push on the gun, in other words.  Quicker and easier just to push on the gun a little, of course.

The velocity variation was a little high for my gun but not the highest I have measured.  The ES was 31 with a 7 fps std deviation.  I've measured higher but most strings have been better than this.  But since it's within my prior experience I see no reason to blame the moderator.  I don't think this sort of velocity variation is any sort of issue.  I like the single digit numbers I typically see with my P35 but I don't shoot many groups as small with it as I do with the Avenger. 

So overall, I am quite pleased with the addition of the Tanto - other than the 5 inches added length and the cost.  But it works. 

[rangerfredbob]

I could give a rats behind about the actual sound for myself or neighbors, it's all about the intended target and how bad of a shot I am in the heat of the moment off hand in precarious situations . If I pull the first shot, with a quiet airgun I can take a second shot, my .22 Marauder spoiled me...

Don't remember if I posted a reply or not but my Buck Rail order came in last weekend and I tried it out, definitely changed the tone of the Avenger, don't know what the numbers would say but the tone changed at least... have yet to test it on a pest yet, I missed one last weekend and spooked it so now it knows I'm a threat... finally shot the Avenger at a target for the first time since just after I got it and it was 3/4" high and 3/4" right so that explains a lot... turns out shooting at black walnuts and calling that good enough wasn't good enough...

[JimD]

I have something that works, at least on my gun, better than my DonnyFL Tanto!  Another member, runner57us, sent me two printed moderators to test.  I test with my cheap Amazon meter 15 feet in front of my gun and slightly to the side.  It sits on a window sill so it is up against the window.  The average of 3 shots with the Tanto on the end of the DonnyFL adapter was 86.6 db.

With John's (runner57us) moderators, I averaged 80.6 and 81db.  The shorter one, that doesn't increase the length of the gun at all, was actually quieter so I left it on the gun.  The longer one increases the length a little but less than the Tanto + adapter.  The readings were not as consistent as with the Tanto - to be fair - but the highest reading was only 0.4 db higher than the Tanto average.  I also checked accuracy with the shorter one and it was at least as good as I got with no moderator or the Tanto.  One group was only about 1/8th center to center at 25 yards.  I would say accuracy did not change but it might have gotten a little better. 

The two biggest downsides of the Tanto in my mind are the cost and the added length for what is, in my mind at least, an already long gun.  This solves both.  I worry a little about the printed moderator on my Prod from Buck Rail because it is almost 5 inches long and plastic.  John put longer threads on his longer one in apparent recognition of fragility but the short one I am using just looks like a different end cap on the gun - nothing to worry about from a fragility standpoint.  Both the long and the short seem to extend down over the end of the barrel like the DonnyFL adapter.  I had no clipping issues with either of John's moderators (or the Tanto). 

If you are interested in this option please send John, runner57us, a personal note.

Jim

[DMR214]

Glad to hear you having success with that bark (roof roof)
That sounds interesting since I too, hate the added length, except I have the sumo that i opted for in .25/30. Fill probe barley fits by a hair. I'm only at 41fpe so those printed ones shouldn't have as much stress as 48. I've only taken squirrels and birds really and the ones I missed definitely flew off. I once went to visit family in west tx, in a sort of residential area, I shot a pigeon with a domed 25gr. The shot was quite  the impact on the bird was so loud I got scared lol. T Thought someone heard it , the echo of the buildings around made it sound louder. So in this case the sumo was pointless cause the rest of the birds still flew away, just added length to the rifle.
But I'll be checking out runner57 for sure , thanks for that heads up.

[subscriber]

So, earlier in this thread I suggested to Peter (Soupyyy) that he incorporate a multistage air stripper into his Avenger and Liberty LDC designs.  The typical air stripper has a fixed standoff distance from the muzzle that is a compromise between maximum shroud filling, and maximum turbulence reduction as the pellet leaves the muzzle.   Optimizing the latter should improve grouping ability.

Placing the air stripper nearer the muzzle protects the pellet from the initial uncorking blast and turbulence, but does not have enough flow area to take full advantage of the shroud volume.  Placing the air stripper far ahead of the muzzle sends more air to the shroud, but allows the pellet to travel through "hostile air" over a longer path and over more time.  Buffeting the pellet is not generally good for tight groups.

In any event, rather than making more suggestions, I decided to actually design an LDC for the Avenger and Nova Liberty (the latter has a 28 mm longer shroud face to muzzle distance).   My design incorporates both a close and a far air stripper, to combine minimized muzzle turbulence with a large air flow area towards the rear of the shroud via two sets of independent passages.

There have been a few versions actually, that Peter printed and tested.  The attached images are for the latest iteration.  It has a 30 mm OD section projecting 104 mm ahead of the shroud.  I dimpled the tube wall heavily to save weight (about 80 grams), but making the dimples shallower might look better, and stiffen the tube wall. 

In any event, Peter performed his usual sound test protocol to measure 77 and 88 dB with his Liberty, shooting Croman 14.3 grain .22 pellets at 16 and 29 FPE respectively (low and high power settings).   

Anyway, I thought that with the work Peter put into this, the results should be shared.