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Sound Suppressor Discussion
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PostPosted: Sun Jul 08, 2018 6:41 pm 
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This is for bolt action 300 blackout

I see one theory that starting from the muzzle, you want a large blast chamber, then progressively smaller chambers. The theory is that you need to start with a big space to absorb the high energy blast. As energy is shed in each chamber, however, you can get away with having a smaller chamber.

Say a 60k psi explosion in the chamber becomes 50k psi in the barrel. Then drops to 40k psi in the first chamber, then 30k and so on, until finally it's 10k in the last chamber and that drops to the atmospheric 14 psi at sea level.

But maybe the opposite theory is the correct one. This thinking is that a large blast chamber is bad, because it means you're going from the high pressure in the barrel, to a large chamber and that means a sudden big drop in pressure. And a sudden big drop in pressure is explosive. It's exactly what you're trying to avoid, that sudden drop from 50k in the barrel to outside atmospheric 14psi for example.

This second type of thinking says that a big blast chamber doesn't help protect the baffle from the erosion of the blast. Whether the first baffle is two inches away from the muzzle or one inch it's still going to get pretty well the same blast. Some microscopic erosion is still going to happen.

No, don't worry about metal erosion, instead focus on sound reduction. You want the chambers to get progressively larger, as you want to avoid large drops in energy. The first few chambers are small as you don't want 50k to suddenly drop in half all at once. but as the pressures drop, you can give them larger chambers without the drop being explosive. Eventually you're going to make the drop to the biggest chamber of them all, the entire outside atmosphere.

Or are both theories bunk and chambers may as well be same same all the way down the pipe?

Very curious about all this.


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PostPosted: Sun Jul 08, 2018 10:46 pm 
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A couple of small things I'm going to note.

Blackout subs don't reach near 50k psi even at the chamber and certainly not at the muzzle. But I'm going to assume that you're just using 50k as a reference.

Usually, a large expansion chamber exhibits a lot of first round pop. The more oxygen there is to burn when the bullet comes uncorked, the louder that first round pop is going to be because the pop is the gasses burning up the oxygen present in the blast chamber. That's why subsequent shots are more quiet (less oxygen is present for a duration).

And as a stark contrast, too short of a chamber drastically increases back pressure. This is more evident in semi's than bolt actions, but it does still increase fouling somewhat regardless of the action.

There has been a lot of discussion in the past on the forum where we've even talked about having a larger secondary expansion chamber a couple of inches into the stack based on slow motion still photos that show that the expanding gasses tend to mushroom at around that distance.

And certainly, since you only get one shot at making parts, many folks will make varying lengths and arrange and rearrange them in an attempt to see what works.

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PostPosted: Mon Jul 09, 2018 12:14 am 
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Ok, here is the idea. I think I agree with you that progressing chamber size is the way to go. What does everyone else think?


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PostPosted: Mon Jul 09, 2018 12:19 am 
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The videos show red heat cans at the middle. so that is where the heat buids up. so maybe small chambers at first then bigger then smaller.


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PostPosted: Mon Jul 09, 2018 12:39 am 
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I think that thermals show the most heat several inches into the bore is because of the mushroom (secondary expansion) that I mentioned earlier.

What is the recurring theme you notice here? :

https://www.google.com/search?q=slow+mo ... 1110113596

Image

Image

Image

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Image

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PostPosted: Mon Jul 09, 2018 9:36 pm 
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I suspect that what we see in the photos is the period just after the burning powder stops releasing the oxidizer in the gunpowder. Ideally, the 'gunpowder" or propellant, will have just enough captive oxygen to support efficient combustion, but with the super-heated air exiting the muzzle, and the presence of lots of free ( Not compounded) oxygen in air, the remaining super-heated propellant has enough energy to create the bright flash of a secondary extreme oxidation event.


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PostPosted: Mon Jul 09, 2018 11:16 pm 
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But does it matter?

That's what I'm curious about.

Let's say the force is stronger in a particular area of the can. But why does that mean the chamber in that area should be large. Or small. Online I see theories going in both directions.

Does it matter, and why?

I honestly don't know.


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PostPosted: Tue Jul 10, 2018 12:06 am 
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kohalajohn wrote:
But does it matter?

That's what I'm curious about.

Let's say the force is stronger in a particular area of the can. But why does that mean the chamber in that area should be large. Or small. Online I see theories going in both directions.

Does it matter, and why?

I honestly don't know.

Because a larger chamber with no blockages, such as a baffle, means there is less exerted pressure on the walls of the suppressor. It also means that by the team the propellant gasses reach the first baffle they would have cooled more and combusted more generating less pressure and lgoing slower increasing the life of the baffles.
As for baffle spacing after that I can’t really say but I’m guessing more baffles equals more suppression as the pressure gets lower and lower.


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PostPosted: Tue Jul 10, 2018 12:09 am 
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John A. wrote:
A couple of small things I'm going to note.

Blackout subs don't reach near 50k psi even at the chamber and certainly not at the muzzle. But I'm going to assume that you're just using 50k as a reference.

Usually, a large expansion chamber exhibits a lot of first round pop. The more oxygen there is to burn when the bullet comes uncorked, the louder that first round pop is going to be because the pop is the gasses burning up the oxygen present in the blast chamber. That's why subsequent shots are more quiet (less oxygen is present for a duration).

And as a stark contrast, too short of a chamber drastically increases back pressure. This is more evident in semi's than bolt actions, but it does still increase fouling somewhat regardless of the action.

There has been a lot of discussion in the past on the forum where we've even talked about having a larger secondary expansion chamber a couple of inches into the stack based on slow motion still photos that show that the expanding gasses tend to mushroom at around that distance.

And certainly, since you only get one shot at making parts, many folks will make varying lengths and arrange and rearrange them in an attempt to see what works.


I think that the secondary explosion is just what another person, secondoray combustion in the air from the oxygen and also greater surface to be exposed to said oxygen(some air is also sucked into the barrel and some is propelled out which could also play a role with this mushroom cloud like effect) but I also think it’s a “wave” created by the bullet disturbing the air around the propellant gasses.


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PostPosted: Tue Jul 10, 2018 12:17 am 
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I think the mushroom cloud we see is just the same as the mushroom cloud from an atomic nuclear explosion. At first, the energy is so high that it rushes straight up, so fast that no sideways expansion happens. The low pressure of the air around it is of little concern.

Then after travelling some distance, after pushing against atmosphere, it slows down. Now, having less energy, the high pressure of the rising column is tempted to rush out sideways to fill the low pressure atmosphere around it. So it mushrooms out sideways.

Same thing with guns. Yes?


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PostPosted: Tue Jul 10, 2018 12:32 am 
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kohalajohn wrote:
I think the mushroom cloud we see is just the same as the mushroom cloud from an atomic nuclear explosion. At first, the energy is so high that it rushes straight up, so fast that no sideways expansion happens. The low pressure of the air around it is of little concern.

Then after travelling some distance, after pushing against atmosphere, it slows down. Now, having less energy, the high pressure of the rising column is tempted to rush out sideways to fill the low pressure atmosphere around it. So it mushrooms out sideways.

Same thing with guns. Yes?


Yes, that’s also makes sense however with nuclear mushroom clouds the mushroom is because the air above is less dense so more expansion plus the other side of the explosion hits the ground, similar with guns with the ground the barrel and the Lower denser air simply time allowing it to cool and be able to expand instead of being a stream.


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PostPosted: Wed Jul 11, 2018 8:50 am 
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kohalajohn wrote:
I think the mushroom cloud we see is just the same as the mushroom cloud from an atomic nuclear explosion. At first, the energy is so high that it rushes straight up, so fast that no sideways expansion happens. The low pressure of the air around it is of little concern.

Then after travelling some distance, after pushing against atmosphere, it slows down. Now, having less energy, the high pressure of the rising column is tempted to rush out sideways to fill the low pressure atmosphere around it. So it mushrooms out sideways.

Same thing with guns. Yes?

Mushroom clouds are caused by heat pulling the air up, not from explosive propulsion. They look similar, but gunshot "mushroom clouds" work differently.

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PostPosted: Wed Jul 11, 2018 11:23 am 
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It seems to me that the 'mushroom expansion' of gases in front of a muzzle is from friction with the air and has no direct correlation to gas flow inside a suppressor. The baffles and containment totally change the physics involved, vastly speeding the process of expansion and cooling much closer to the muzzle than open air.

I attribute (admittedly with nothing more than engineering training in heat transfer and a basic understanding of the physics involved) the thermal cooling/coolness (relatively) of the mount end of a suppressor shown in the pics above to extra metal in the endcap having a higher thermal mass than the (relatively thin) mid section, so it heats up much slower. That would be easily confirmed (would love to see it done, haven't and don't have the instruments to do it myself) by watching the thermal process after shooting to see if that mount endcap area stays hotter longer and the middle that heats up faster also cools faster. Finally, the endcap area is also not in the direct impingement path of full velocity muzzle blast like the baffles are, which would also tend to slow its heating rate.

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PostPosted: Wed Jul 11, 2018 2:00 pm 
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I jumped-out when the talk turned to nuclear weapon metaphors...I think the secondary oxygenation event happens irrespective of whether there is a suppressor or not, its just more visually clear in the suppressor tube. Its also a good indication that the burn rate of the powder is not thermodynamically optimal (Too much "boom juice") Having the volumes of the various chambers inverse respective to the "normal" sequence would result in higher back-pressures ( With back-blast and chamber pop consequences for semi and full auto applications) increased tube and baffle wear, and god-awful suppressor heating. Would it "work"? ( Make the suppressor quieter) I don't think so, although my education in thermodynamics is a little bit dated. I suppose you could a) Model it, or b) drive threads into the wrong side of a monocore and try it both ways. I can't think of an elegant proof or argument for why the smaller chambers should be first. I suspect that the optimal gas cooling ( And the combined laws of gasses suggest that temperature IS pressure) would be in a series of spaces following a Fibbonachi sequence. Why? Because everything else in Nature tends to go that way. Mushroom clouds, really?


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PostPosted: Wed Jul 11, 2018 2:14 pm 
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alordnapa wrote:
I jumped-out when the talk turned to nuclear weapon metaphors... Mushroom clouds, really?


Nuclear weapons metaphors are only appropriate when discussing 380 pistols. :lol:


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PostPosted: Wed Jul 11, 2018 2:44 pm 
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Lots of people (me included) have used variable length spacers in their cone baffle form 1 cans. I can (and plan to...just finished it) reverse my stack from the traditional (and industrial standard) large to small and see what happens. It is on a 22lr bolt rifle, so it will be all muzzle noise, no issues with semiauto chamber pop or the like. What I lack is a good microphone to record or measure sound, so that is all subjective. I think I remember other members performing this exercise and finding out the traditional setup was superior but that is an old memory...

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PostPosted: Wed Jul 11, 2018 4:30 pm 
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jlwilliams wrote:
alordnapa wrote:
I jumped-out when the talk turned to nuclear weapon metaphors... Mushroom clouds, really?


Nuclear weapons metaphors are only appropriate when discussing 380 pistols. :lol:



Hey, I love nuclear weapons, don't get me wrong! I can think of at least six places where they can be used profitably right now!


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PostPosted: Thu Jul 12, 2018 2:33 am 
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cdhknives wrote:
Lots of people (me included) have used variable length spacers in their cone baffle form 1 cans. I can (and plan to...just finished it) reverse my stack from the traditional (and industrial standard) large to small and see what happens. It is on a 22lr bolt rifle, so it will be all muzzle noise, no issues with semiauto chamber pop or the like. What I lack is a good microphone to record or measure sound, so that is all subjective. I think I remember other members performing this exercise and finding out the traditional setup was superior but that is an old memory...


I did the same with my first F1 can, and found that with my can (using a variation on cone baffles) and subsonic rifle loads, the baffle spacing was a balance of first round pop vs suppression level of subsequent shots. Larger spacing near the rifle muzzle gave more first round pop but a quieter and deeper tone for subsequent shots.


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PostPosted: Fri Jul 13, 2018 1:30 pm 
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I did the same with my first F1 can, and found that with my can (using a variation on cone baffles) and subsonic rifle loads, the baffle spacing was a balance of first round pop vs suppression level of subsequent shots. Larger spacing near the rifle muzzle gave more first round pop but a quieter and deeper tone for subsequent shots.[/quote]



The first round pop thing is interesting. I know the standard accepted theory for it is oxygen depletion.

The oxygen theory seems strange to me. Yes, some combustion can happen inside cans, using up oxygen, but as gases are blown through the can they are replaced with outside air, air that has normal 21% oxygen.

Same as in the barrel, yes combustion depletes oxygen, but those gases are immediately replaced with outside air at 21% oxygen. The second shot does not suffer from a lack of oxygen in the barrel. The second shot is the same.

In our form 1 cans, the second one is quieter. If it's due to the can heating up, that means that the dense cold air means loudness.

Loudness is from a dramatic drop in gas pressure. The rapid expansion sends out a shock wave. The fact that a bigger first chamber means more loudness makes sense as you're making a more dramatic pressure drop. from the 30k psi in the barrel to way down to say 5k psi. We only make the first chamber large as we're trying to protect the first baffle, or we've got a gas impingement AR and we're trying to lessen gas blowback in our face. It seems agreed that we pay a price in more loudness.

Intuitively I'd think cold dense air would be better it means a smaller drop in pressure. But the opposite is true.

Thoughts?


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PostPosted: Fri Jul 13, 2018 2:06 pm 
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Your definition of immediate and mine are probably a bit different. A suppressor on a gun that had the bolt closed is not passing substantial volumes of gases when 'idle'. The only path for replacing the combustion gases in the can from the previous shot is through the exit hole, and that will take substantial time via diffusion and a very small amount of physical circulation. It is basically an exit only single hole into a closed volume. Want to experiment? Shoot two through a cold can and listen/monitor for FRP. Use air compressor down bore to blow out bore and can. Shoot 2 more and monitor. Tape end of can and wait (closed bolt) for cooling. Remove tape and shoot 2 more. Wait 1 minute and shoot 2 more. Compare FRP between the 4 scenarios. I would expect the first 2 and the last 2 to be very similar if it is a oxygen level issue...with the first 2 having significant FRP and the last 2 having almost none.

For a long time I swore the FRP phenomena was just the human ear hearing the first report as loud compared to a quiet background and the second shot as less loud after the first 're tunes' the ear for higher noise levels. Instruments don't suffer this perception effect and I have seen reports that they also record FRP as real in some cases, so I have to conclude it is a real thing.

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PostPosted: Fri Jul 13, 2018 2:31 pm 
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combustion does deplete the oxygen.

But remember, oxygen is flammable and is a necessary part of combustion.

Fill the can with something like nitrogen or other heavier gas, and the sound will be less.

Fill the can with gel ablative and it does reduce some of the overall area inside of the baffle and makes it more quiet.

So, I believe that's why it's pretty much universally agreed that oxygen (and the amount of it) is the biggest culprit of frp.

Cooler air makes engines more efficient. jet engines, car engines. But you're not trying to make the propellant more efficient.

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PostPosted: Fri Jul 13, 2018 4:19 pm 
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OOOh, my Chemistry Professor would eat you liver with beans and a nice Chianti if he heard you suggest that only Oxygen can support combustion. Unfortunately, he died in a suspicious fire...


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PostPosted: Fri Jul 13, 2018 11:22 pm 
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OMG, it's both.

Both oxygen and coldness are creating frp.

The first blast is in a more oxygen rich environment as the atmospheric air in the can is replaced with a mess of exhaust gases, so a bit less oxygen for the second shot.

But it's also the cold in the can before the first shot. Cold air is more dense and more dense means more of everything including more oxygen. More oxygen means a more robust explosion. The way turbo charging your can engine does.

Would be interesting to shoot some pure oxygen into the can. That would probably make it louder. And putting your can in the freezer for a while.

There was a thread on coldness and loudness. viewtopic.php?t=5837


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PostPosted: Sat Jul 14, 2018 7:11 pm 
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You know, I can't even tell if this topic is real or just an insanely over-thought troll conspiracy...


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PostPosted: Sat Jul 14, 2018 8:45 pm 
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Seriously, not trolling. Having fun. We're talking about the laws of thermal dynamics and the classical physics problem of turbulence.

Very mind bendy.

Well, perhaps more so when my mind has been bent a bit by a gin and tonic before I read this thread....


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