Taming the bark of an Avenger

[WhatUPSbox?]

I noticed someone posted a design on thingiverse in the last few days. Might be interesting to compare features.
https://www.thingiverse.com/thing:5327234

[subscriber]

Thanks, Stan

That thingy has four narrow air stripper slots communicating four small holes, evacuating one air stripper wall.  The internal structure is also very different.

You are giving me an idea.  It is more to prove a point...

[JimD]

Still getting my head around air strippers.  Subscribers posts help.  I did notice that putting the baffles in the shroud of my P35 backwards did not work as well for sound suppression.  I don't remember if I shot any groups.  It has thin aluminum baffles that are kind of cone shaped and wants them pointed back towards the barrel.  Like an air stripper I think.  That may be one of their functions. 

The other thing I noticed is a small improvement in sound suppression with a half curler/baffle followed by a full curler/baffle as compared to three half curlers or a full curler plus half curler.  Kind of a small then larger chamber for air expansion. 

I tried peaking inside the moderator/shroud insert John sent but I couldn't tell much about it's design.  It works well but did not seem to reduce group size much if at all. 

The moderator on my Prod seemed to reduce group size noticably.  I wonder how it is made.  Buck Rail. 

As inexpensively as it seems to be for a manufacturer to make their rifles about 20 db quieter I wonder why they do not just do it.  Seems like it only takes a few dollars of plastic to have a pretty big effect.  Avenger shroud is particularly poor in my opinion with the barrel end unsupported and no attempt at baffling to reduce noise. 

[subscriber]

Jim,

There are many approaches that work.  Some of them appear contradictory.  For example, "baffles space lengths must vary, or a resonance will be set up".  Yet, the benchmark for quiet (Marauder) uses so many fixed length simple conical baffles. 

The whole system matters, rather than some detail.

Getting air that is flowing forwards from the barrel muzzle to reverse flow into the rear of the shroud takes some doing.  All "air strippers" work to some degree.  Some of them are rudimentary and look almost cosmetic, rather than optimized within the space available.   Almost as if a marketing box has been ticked off, rather than any real engineering being done. 

An example of this marketing driven design:  Some muzzle loading rifles sport fluted barrels.  The flutes are only just deep and wide enough to be seen.  The amount of weight savings is negligible, so being visible is the point.  Only snag is, flutes don't belong on a muzzle loading rifle barrel.  If you want visual appeal, make it octagonal .

All I can advise you is to try different configurations and see what works.  There are a number of reasons why a longer or shorter chamber LDC works.  It does not matter if the common explanation for it is correct or not.

Are you looking for a flush mounted shroud filler for your Avenger?  Or can you tolerate some projection out front?  If any, what is the max diameter allowed for the forward section?   I have designs for the Avenger that are "flush", and that project out 4" from the shroud.  Below are one version of flush mounted shroud filler for the Avenger with a two stage air stripper.  All you need is the ability to print your own, or to get someone to print it for you.

[swoab47]

Jim,

There are many approaches that work.  Some of them appear contradictory.  For example, "baffles space lengths must vary, or a resonance will be set up".  Yet, the benchmark for quiet (Marauder) uses so many fixed length simple conical baffles. 

The whole system matters, rather than some detail.

Getting air that is flowing forwards from the barrel muzzle to reverse flow into the rear of the shroud takes some doing.  All "air strippers" work to some degree.  Some of them are rudimentary and look almost cosmetic, rather than optimized within the space available.   Almost as if a marketing box has been ticked off, rather than any real engineering being done. 

An example of this marketing driven design:  Some muzzle loading rifles sport fluted barrels.  The flutes are only just deep and wide enough to be seen.  The amount of weight savings is negligible, so being visible is the point.  Only snag is, flutes don't belong on a muzzle loading rifle barrel.  If you want visual appeal, make it octagonal .

All I can advise you is to try different configurations and see what works.  There are a number of reasons why a longer or shorter chamber LDC works.  It does not matter if the common explanation for it is correct or not.

Are you looking for a flush mounted shroud filler for your Avenger?  Or can you tolerate some projection out front?  If any, what is the max diameter allowed for the forward section?   I have designs for the Avenger that are "flush", and that project out 4" from the shroud.  Below are one version of flush mounted shroud filler for the Avenger with a two stage air stripper.  All you need is the ability to print your own, or to get someone to print it for you.

Would you be willing to share the stl file via pm for that shorty moderator??

I have a friend that has a printer.

[subscriber]

Would you be willing to share the stl file via pm for that shorty moderator??

I have a friend that has a printer.

Sure.  I will need an email address.

[subscriber]

Based on feedback about my Short Avenger LDC design above, a user commented that effective air stripping was his main priority.  So, I created a four stage air stripper version of the LDC:

It has the same two stage air stripper feeding the rear of the shroud.  Then, the two forward baffle chambers have been modified with air strippers feeding air vents on the front face of the device; rather than attempt to contain air in the chambers, and then more slowly vent all of it from the bore axis.

If there is any interest in printing this four stage air stripper device, and trying it out, PM me your email address and I will send you the STL.  See images below.

[WhatUPSbox?]

Interesting, The last image looks like there is a converging section as the first element. Am I misunderstanding the view? I don't see it in the other section views.

Update: never mind, I think I understand that view now.

[subscriber]

Hi Stan,

The way the first stage vents blend into the structure looks odd, in section.  Looking at the odd aspect in other views usually helps.  When I am in a hurry, the images I grab and post may not be as clear as I would like. 

Perhaps sections views from behind might help; starting with the whole part:

[WhatUPSbox?]

Thanks. Once my morning coffee kicked in I understood the section. Now I'm looking for the acoustic features in the front vents that disguise the shot sound as a (toot, kazoo, sneeze..?)

[Outer_Rondacker]

Did you modify the threads any from V3? We have printed Pablos from thingiverse and it threads fine. I still cannot get yours to thread no mater what we do. I would love to run the one you have designed but like I said it will not thread. Please send the stl for V4 to my email and I will give that one a try. I really a appreciate all the work you for us. Thanks Outer.

[subscriber]

Adding a clarinet reed made from a plastic straw to each vent may be possible.   Then you get the sound ducks make when they are shot at

[subscriber]

This is a conundrum.   I was using the nominal specs for an M24 x 1 mm pitch thread to generate a thread in Solidworks.  User Soupyyy was having trouble with it being tight in his Liberty shroud threads.   His remedy is to chase the threads with a die, removing the excess printed material.

 So, I modified that thread in my design by setting the pitch circle diameter smaller than nominal by .05 mm.   The threaded parts I have designed in the last week all have the smaller pitch circle diameter.  Now, it is possible that I am corrupting something else, and rather than helping, I could be making things worse.  All I know for sure is that the thread has an exactly 1 mm pitch, with an OD and pitch circle that are smaller than nominal.  If you measured the OD as 0.2 mm too large as printed (for example), I could reduce the thread form to adjust for that. 

In fact, perhaps I should leave the pitch circle diameter at nominal, and just take the little triangle that is the thread form, and give it a "shave"...  My problem is that I am not getting feedback, beyond "it does not fit".  So, I don't know if there is a lack of space between threads, or if the diameter is wrong.

Please measure the LDC body and thread OD and report back.  That will tell me if there is a percentage error.  Also measure the overall length of the LDC.  The space between threads may be slumping closed and a thread flank shave may be better than a diameter reduction.

Does the shroud thread foul on the LDC tube section before you get to the the threaded section?  The tube design diameter I used is 22.4 mm.

Please email me the thinyverse part that prints its thread correctly.  Or; email, or post a link to the part on GTA.  I will see if I can learn something from that.  If I can convert it to a STEP file, I can imbed the threaded section in my design, in place of my thread.

I can also send you STL files for a series of short threaded tubes, that can be printed faster than the whole LDC.  Each sub-part would have a different pitch circle diameter.  Once you identify which size provides the desired fit, I can imbed those in the LDC designs.  Or after thinking about it, a shaved thread form with more space for the matting thread flanks.

Did you modify the threads any from V3? We have printed Pablos from thingiverse and it threads fine. I still cannot get yours to thread no mater what we do. I would love to run the one you have designed but like I said it will not thread. Please send the stl for V4 to my email and I will give that one a try. I really a appreciate all the work you for us. Thanks Outer.

[WhatUPSbox?]

I think he is talking about the thingiverse part linked in reply 60 above

[subscriber]

I think he is talking about the thingiverse part linked in reply 60 above

I believe you are correct, Stan:  https://www.thingiverse.com/thing:5267572

[Outer_Rondacker]

Yes that is what I was talking about.

[subscriber]

I found probable cause for my thread not fitting.  The root diameter was oversize by 0.0055".

I have sent the STL file for a short threaded cylinder with that problem corrected, to Outer_Rondacker.  If he confirms the improvement, I will replace the thread on the LDC and air stripper designs.  Else, I will make more adjustments.


I just opened the thingyverse thread and can confirm that it uses the same root diameter as I have on my corrected thread.  However, I notice that the thingyverse thread form is more "open" to accept the mating thread form - or tolerate printed thread distortion.  I am in the process of quantifying that now.  It may be that flank interference is a second factor that needs help.

[subscriber]

I created four different M24 x 1 thread fit test file parts.  The STL files are attached to the nest four replies to this thread.   Attached that way due to total attachment size limits.

It would be great if someone could print these thread forms to find the best fit in the Avenger shroud thread.  By best, I mean not just easy to screw in, but strong.  The Thingyverse thread looks "easy", but weak, compared to the other variants below.

I think they need to be printed one at a time to create the strongest, most representative thread form per 3D model.

I can email the STL files to whomever wants to try them (but the files are also attached to the replies below).  All have the same 22.6 mm root diameter - including the Thingyverse model:

The thread form triangles below are for a thread with its long axis pointing up / down.  Perpendicular to the image of modeled threads show in the second last attachment.

Test prints look like threaded tube shown in last rendering, below:

[subscriber]

Attached:  STL for above M24 x 1 nominal test piece:

Looks like threaded tube shown directly above, in previous post.

[subscriber]

Attached:  Shaved m24 thread STL version 1: