Moderator Design

[heavy-impact]

I don't like moderator designs that let a lot of air pass the projectile or strip air from the projectile on a bias that has the same effect as a crooked crown. In my testing the most effective design by far strips the maximum amount of air away from the projectile and takes it on a longer path than the projectile allowing the pressure reduction to be most linear with projectile travel. With a really efficient moderator you still hear the air leaving for a fraction of a second after the shot.
You can see that the projectile almost seals the air stripper baffles with a .6mm/.024" clearance in .30 caliber stripping the air away and giving it time to expand without passing the projectile. If you don't delay the high pressure air it will pass the projectile causing turbulence and more sound to escape.
Ideally you want the projectile to leave the moderator before any of the air that propels it thus allowing the air pressure to drop to a level that creates an acceptable report.
The straight wall height of the baffles can be tailored to suit projectile length so that the next chamber is sealed before high pressure air can pass the projectile if you'd like but I don't find it necessary.
These are the modular 3d printed baffles I used in my latest build. When they're in the can there's another chamber before the projectile enters the first baffle.

You can click the pics twice for the largest version.
Next pic is the main baffles.

Below is the exit baffle. The aluminum end cap hole is large so if I ever had clipping it would only damage the plastic baffle and not the aluminum cap.

This moderator is 194mm long and 40mm id. Cost me less than $20 total to make and a couple hours of my time. Crazy priced mono core suppressors do not come close in sound reduction. I can't believe what people pay for horribly inefficient chinese manufactured monocore suppressors from the big names in this business.

Please post your thoughts on this subject.

 

[subscriber]

I agree with your bias against cross-flow baffles, based on the value of a square crown analogy.

Setting up the baffle geometry so the projectile acts as a moving plug helps efficiency. Now, your image shows a long projectile, so the leak reduction effect would be reduced with a shorter projectile. There are trade offs with how close the baffles are; and how many baffles there are. Too many baffles use up expansion volume, but if you use that volume more efficiently you may still be ahead.

Slanting the baffles beyond an included cone angle of 60 degrees helps the air "peel off". The "best geometry" depends on the outer tube ID. Often a customer will declare that they want a tube that is larger or smaller than what is optimal, if you consider factors such as the projectile length. You can compensate to a degree by making the cylindrical section of each baffle bore longer or shorter. Or by changing the baffle cone angle.

If you can ensure good barrel bore to moderator bore alignment throughout the shot cycle and projectile travel (not just before the shot), then such tight baffle bore to projectile clearance should help efficiency. Else you might be right on the edge of clipping. Some misalignment is dynamic, and is connected to barrel harmonics.

Ultimately, handsome is as handsome does. There can be few arguments against a robust system with demonstrably good results. If your consider that TKO moderators work surprisingly well, despite their small OD, and their flat baffles; then that suggests there is more than one way to design a functional moderator. Even knowing that, flat baffles don't make as much "sense" to me as conical baffles. Also, flat baffles and are not self supporting like conical baffles for 3D printing. That said, a few lengths of tubing and a bag a washers may be "too simple", but it works.

 

[heavy-impact]

That said, a few lengths of tubing and a bag a washers may be "too simple", but it works.

The problem with tubing and metal washers is that it's an old powder burner design that has the potential to cause you problems from an overzealous LEO.
To remain legal for airgun use the design is not supposed to support powder burner use (avoiding the common term). Printing the baffles from thin plastic has a better chance at satisfying the intended use requirement.
I turn my housings on a mandrel to maintain centerline and print the baffles on a super accurate Bambu Lab X1 Carbon printer so they take a couple pounds of pressure to press into the aluminum housing. This helps keep everything in line to prevent clipping. The AEA Challenger .30 requires 100+ grain slugs to get maximum energy output hence the longer slug in the picture.

 

[subscriber]

The funny thing is that in another thread, I was told that fully printed moderators are too fragile. A steel outer tube may be too robust (and heavy). Opinions seem to vary about what makes an airgun moderator legal and when it crosses the line: https://www.airgunnation.com/threads/moderator-suppressor-rules.1303628/

 

[heavy-impact]

The funny thing is that in another thread, I was told that fully printed moderators are too fragile. A steel outer tube may be too robust (and heavy). Opinions seem to vary about what makes an airgun moderator legal and when it crosses the line: https://www.airgunnation.com/threads/moderator-suppressor-rules.1303628/

My housing is 6061T6 aluminum. The whole thing is lighter than an equal size aluminum monocore.

 

[heavy-impact]

I've been thinking that a rear extension that goes over the barrel and screws on to the rear of the moderator with holes added could increase performance by acting like a shroud and greatly reduce pressure before the baffles. Please overlook my sloppy modeling.

 

[OldSpook]

The problem with tubing and metal washers is that it's an old powder burner design that has the potential to cause you problems from an overzealous LEO.

No. I think you have misread the courts findings on that.

To remain legal for airgun use the design is not supposed to support powder burner use (avoiding the common term).

The findings in the appellate case respect the intent of the builder and explicitly refuse to consider the construction method.

Printing the baffles from thin plastic has a better chance at satisfying the intended use requirement.

Clearly true.

I turn my housings on a mandrel to maintain centerline and print the baffles on a super accurate Bambu Lab X1 Carbon printer so they take a couple pounds of pressure to press into the aluminum housing. This helps keep everything in line to prevent clipping. The AEA Challenger .30 requires 100+ grain slugs to get maximum energy output hence the longer slug in the picture.

Since you are making the vertical sidewall and using it to adjust separation wouldn't it be easier to print it in one piece?

 

[heavy-impact]

Since you are making the vertical sidewall and using it to adjust separation wouldn't it be easier to print it in one piece?

I like to print them separate to be sure each part is perfect and it makes the design modular so if your first baffle is starting to wear from the higher pressure hitting it you can swap out the one. If you clip the last baffle you can swap it for one with a slightly larger hole. I feel like I get a more accurate prints also by keeping the parts short relative to their footprint on the build plate. Thank you for educating me on the legal developments.

 

[FLgunner]

I've been thinking that a rear extension that goes over the barrel and screws on to the rear of the moderator with holes added could increase performance by acting like a shroud and greatly reduce pressure before the baffles. Please overlook my sloppy modeling.

View attachment 420902

I really like this idea! I have a new 600mm .177 barrel coming for my Crown. Since I shoot in my backyard, I need a good moderator, but don't want to add any more overall length to the airgun than I have to. A design like this that has a rear chamber over the Crown Shroud with a nice baffled end would be perfect!

 

[subscriber]

I like the rear tube extension behind the barrel muzzle because apart from enlarging the primary blast chamber's effective volume, the rearward length can be used to take bending moment off the barrel stud. More important for a longer heavier moderator.

If the reflex length is short I just have holes feeding the rear volume, similar to what you have shown. If the reflex length is more than perhaps 4", I often use a curved air stripper to help feed that space. But such an air stripper uses up moderator length, so if that is in short supply, I would rather use length ahead of the barrel muzzle for conventional air strippers and expansion chambers, than a fancy curved stripper feeding a small reflex volume. The barrel itself takes up considerable air volume in the reflex space, so the difference between barrel OD and shroud ID should be considered.

There a several basic diverter / air stripper designs. The simple flat wall baffle ahead of the muzzle with side ports feeding the reflex volume works, even for smaller ID moderator or shroud tubes. The fancy curved air stripper designs require more radial space to fit in a reasonably large radius, so make more sense with larger tubes; and no sense with small ones.

 

[subscriber]

I like reflex moderators that have the rear section at least half the length of the section ahead of the barrel muzzle. Even if the primary consideration is bending moment reduction at the barrel stud and shoulder.

I was helping a guy design baffles for his Vixen that has a shroud OD of 50 mm, and huge rearward length. I could not understand why it was still so loud, until I realized that having such a large tube fed via very little restriction simply had the primary blast pressure wave run into the wall at the rear of the shroud, and then bounce back out towards the front. I had started with a three stage reflex air stripper, sending air into the huge reflex shroud, and that was part of the problem. Ended up with only the first air stripper feeding the rear of the shroud so the others in front of the muzzle could be conventional cones, and they system worked much better. So, when we optimize for one aspect (rear shroud filling in that case), we may compromise another factor. Testing will tell.

The above guy has gone on to print the outer walls of his insert tubes using gyroid infill, and is experimenting with TPU flexible materials. He has had promising results with both. He now understands enough to experiment on his own. Here is an indirect reference about gyroid foam, pointing back to "the guy" I was taking about: https://www.airgunnation.com/threads/porous-moderator-design-test.1302564/

Some of my designs work better than expected; some worse. I try to understand the reasons for both; to reuse the principles yielding "better", and avoid the "worse". Of course, in systems, it is not just one principle or another. It is the way they interact.

 

[heavy-impact]

I've been messing around with a design to 3d print in one piece while being short and having good volume for expansion.
This one is intended to slip into a shroud and/or over a barrel end rather than screw on but it could be made either way.
The high pressure air expands into the large underside chamber before passing through the first baffle. Length outside the shroud is about 4 inches.
What do you think?

 

[subscriber]

heavy-impact,

I like it. It is a good way to increase the blast chamber volume, without increasing the moderator length. Or making the OD too large.

My only technical concern is about flat sided pressure vessels being less robust that rounded ones. Not a problem if you have a lot of volume to keep the pressure down. Else, here are a few ways I have dealt with that aspect:

Huben - GK1 - taming the bark

It's beautiful!! I like it a lot! ✅ Diameter 2" = 5.1cm Length 4" = 10.2cm Those specs seem to promise "backyard quiet" for 20FPE in .22cal....🤔 I will want one for the Huben GK1 I will buy of the classified when my airgun fund has recuperated from some hard knocks. ✅ And for now...: Can...

www.airgunnation.com

You might find that if you split your large under chamber into 2/3 first and 1/3 second, with a perforated wall between them, the initial surge of air would not just bounce back out, or reverberate back and forth in that long chamber (think of the perforated wall as a depinger of sorts). Some of my designs at the above link might amuse you. The oval offset muffler is particularly effective for its size. It also has an enlarged last chamber, as that affects the tone a lot.

Ultimately, what is simple and works, is good; rather than a one size (recipe) fits all approach.

 

[beerthief]

off setting the moderator in an oval chamber , great ! I always thought of an offset bore with oval baffles , but that would induce turbulence.

 

[denovich]

@subscriber in most of muffler designs (car variety) the inlet/outlet pipes extend some way into the chamber. I think it helps prevent the air from as easily following the pellet through the hole. I suspect the cones in @heavy-impact 's design might be improved by, instead of a truncated cone, extruding the hole a a few mm.

 

[heavy-impact]

@subscriber I suspect the cones in @heavy-impact 's design might be improved by, instead of a truncated cone, extruding the hole a a few mm.

I do that with the length of the cone. I make the cones twice as long as I used to make them. You don't want the projectile traveling through a tube for too long because it hurts accuracy.

 

[subscriber]

denovich and heavy-impact;

You are both correct about long snouts, but it is a matter of scale:

Look at the long nose cones used in the Taipan Veteran, below. Note that the parallel tube diameter has a smaller short cone at its leading edge. So, both goals are achieved: Reaching into the expansion volume; and avoiding a long narrow parallel tube for the projectile to fly through.

I have done something similar with my coaxial design, directly below. It was derived from my offset oval muffler for the Huben pistol - images further down. The latter works rather well in terms of sound suppression and not moving POI or opening groups.

There is more than one way to skin a cat. There is no arguing with real test results. In other words, I would be wrong to tell someone else that their design needs improvement, if it performs better than expected.

So, I make suggestions, that people are free to try, or not.

I have optimized designs to the point where they worked worse than expected. That taught me new lessons about practical limits; not that the principles I understood were wrong. More a case of several principles interact, and that interaction needs to be managed.

My goal is to encourage, not discourage. Even exploring what seem like designs that violate principles, rather than staying with what worked before. For me, that has been offset mufflers, because I have an aversion to asymmetrical turbulence designs. My current view is that asymmetry it is not as injurious as I thought; or that my tube in tube designs are effective at minimizing the effects of asymmetry.

"Long snout" baffles can be seen as tube in tube, if the proportions are right. The inner tube in my designs above has an ID of just 18 mm. This is the tightest I have ever used in a high power PCP application (Huben pistol), and yet it works just fine. Now, that is probably because of the large primary expansion chamber underneath in the oval offset muffler. Ditto for the large concentric primary expansion chamber in the cylindrical derivative design. The cylindrical concentric derivative is as yet untested. But it would seem less challenging that the offset oval.

If you have an idea, try it.

 

[heavy-impact]

denovich and heavy-impact;

............

I believe your design would be more efficient if the first baffle were closer to the barrel and the lower chamber open only to the muzzle. Since the expanding gas will pass the projectile until expansion rate falls below projectile velocity, providing the gas an alternate route long enough to reach that point as soon as possible is most effective. We're just finding the best compromise between thermodynamics and accuracy.

 

[subscriber]

I believe your design would be more efficient if the first baffle were closer to the barrel and the lower chamber open only to the muzzle.

The bulk pressure at the first chamber is high enough that the air wants to travel radially into open space. As the pressure in the barrel drops, it forms more of a linear forwards jet. If the baffle opening is too close to the barrel muzzle, more of that air jet just runs straight down the central bore.

The blast chamber wall is perforated, so that air will pass into the next space while the bulk pressure is high. The holes are intended to damp the flow into the secondary space, and back out of it - rather than slamming in and bouncing out.

I have some designs where the first baffle cone lip is 8 mm from the barrel muzzle. This is part of a two stage reflex airstripper design I did for the Avenger - below. The reason being to protect the pellet from seeing so much of the initial blast at uncorking, and to prevent air from passing the pellet. By itself, a single air stripper that close allows too much air to follow the pellet down the central bore. Hence the second stage reflex air stripper. All of that adds length to the moderator. If there is not enough space then compromises are made.

From my Marauder custom muffler insert below; red arrows point to reflex air strippers. Red lines indicate barrel muzzle section. Baffle cone slant become greater towards front of muffler insert, to encourage air to follow baffle at inside of cone when bulk pressure is low and forward flow velocity is high, after pellet has left muffler. Not just theoretical. This design works rather well:

Here is a clearer image of the reflex air stripper path and barrel seat, on a shorter insert:

 

[steve-l]

I have been making these things for years for myself and have tried most of the things spoken about in this thread. Some of this is old wives tales that offer little or no sound suppression or accuracy value. What does work very well is NO central tube designs, baffles that create separate sealed expansion chambers, large expansion volumes and inverted cone baffles. Flat baffles work and are easier and cheaper to make, but the inverted cone is better because the propelling air has less inertia than the projectile so the air will accelerate past the projectile at barrel exit and if not deflected will interfere with accuracy to some degree. In my experience , more than 3 baffles adds no value in performance and adds only extra construction cost. .Lastly, the use of foam or other space filling material does nothing to attenuate sound.