How quiet can a silenced gun get (PCP air rifle)

[ant3000]

When I first bought my gun I decided to take the silencer off and stuff the barrel into the garden hose, this slowed the gasses down so much it was just about silent bar the click of the trigger. So that made me sure to think it would be possible to make a silencer (with no size or weight restrictions to start with) that made the gun so quiet that you couldnt hear it from the connecting room in my house.

BTW im in the UK and silencers are not needed to be registered with air rifles. Im only using my air rifle cos I can't get access to a real gun. Im hoping that if my test prove that its possible to silence this then someone might let me make a silencer for a sniper rifle, which I think would benefit from serious silencing.

Ive looked into the science of silencers and its all about reducing the pressure difference of the gasses upon meeting, the pop!. Its my theory that there is too much-pressurised gas to slow in a short 6-12" can and if it gets any longer then you run the risk of baffle strikes so the bullet path aperture is enlarged which is counter productive too. Basically the baffles slow the gas flow but the pressure from the shot keep accelerating the gas back upto speed.

Today I finished another silencer for my PCP air rifle with the theory that the amount of baffles reaches a maximum before the back pressure cannot be slowed any more. The stock silencer on my rifle has an over the barrel design and just 3 good cone baffles. So to try and beat this I bought a thick carbon tube and put 3 K baffles at the start then 10 flat baffles after but enlarged the aperture in those by 1.5mm or from 6.5mm to 8mm to avoid baffle stike at longer distances, strikes have ruined previous silencers when I get past 6" from the barrel tip. I was so disappointed to find it only reduced the sound by 1db.

So to get my gun to be even quieter I need to try a silencer with a silencer design that is more than just longer and more baffles or K baffles. I wish I had some sort of benchtop jig that I can test different designs and keep altering without the need to keep rebuilding the whole unit.






Does anyone have an idea for what design they would try to silence this air rifle?





Ive worked out that I can divert about 75% of the gases into a second tube, but ive not done anything with them yet. I could use my one way valve system to trap the gases at high pressures into small spaces, this would avoid the need to silence the gases if they were to expand into the atmosphere causing a bang. Im not sure how restrictive the one way valve system would be on percentage of gas removed.


Any advice on following a sientific process for creating a highly functional silencer would be greatly appreciated?. I figure its not unachievable if stuffing the barrel into my 30 feet of wound up garden hose creates a perfect result, but doesnt allow an accurate bullet.


Cheers
ant...

[doubloon]

Plain old cup baffles work really well on my Benjamin Marauder PCP, just keep stacking them in a row until the noise goes away.

[mbogo]

I see what you did there.

[ant3000]

Im fairly certain that the baffles have slowed the fluid hammer (fastest bit of the gases behind the bullet) back to the average pressure of the gases as they disperse out the barrel and into the silencer chambers, so without more space/volume to disperse into im not going to be able to slow the gases any more. Im down to 81Db which is very sufficient for hunting.

Marauder is the same weapon virtually if I remember correctly. Any chance of a pic?

[a_canadian]

You're not down to 81dB. Give us a break, seriously. 81dB is a quiet conversation. 81dB is dropping a ripe orange onto floor from a foot up. 81dB is what your smartphone SPL app is telling you, and it's a lie based upon the very slow sampling rate of the cheap built-in mic on that device. Or it's a cheap standalone SPL meter. Either way, there's no airgun made delivering less than 100dB, suppressed, ever, period. Even at 5fpe muzzle energy a PCP will be about 100dB. Suppressed. 125dB unsuppressed. Those numbers are standard for 5fpe tuned competition airguns.

[ant3000]

Ha ha, Your right actually, I had to use my iPhone , I’ve got a better one but the battery was dead. I’ll charge it and redo it tomorrow. If memory serves it should be about 103db.

But if you say 100db has never been broken then that seems like positive goal to reach for.
If it wasn’t for the need to maintain bullet velocity and accuracy then sub 100 levels would be easily achieved.

I’m certain that extracting the gases separately from the main silencer is the key to slowing the gases effectively. We’ve got two separate methods of extracting gases away from the bullet (that I can think of) so two separate pressure chambers to deal with gases, the first uses directional flow force to open a gate valve, the force that hits the first baffle maybe diverted in a bent tube that has a hole for the bullet. the second would be utilising the infinite direction of pressure and would be used once gases have slowed.

[fishman]

You do know that this "gas exit right" and "gas exit left" concept is no different than a regular integral silencer, right? Its been done before 1000 times. The o rings are unique but I doubt they'll help a noticeable amount. Try it with and without them installed. Unless your meter costs thousands of dollars, it isn't sensitive enough with a high enough sample rate to come close to catching the peak sound impulse.

[doubloon]

... 100db has never been broken then ...

100dB is routinely broken in the springer class by rifles in the .177-.25 caliber range.

Commercially available suppressed PCPs hover between 100-120 dB for .177-.22 calibers

[ant3000]

You do know that this "gas exit right" and "gas exit left" concept is no different than a regular integral silencer, right? Its been done before 1000 times. The o rings are unique but I doubt they'll help a noticeable amount. Try it with and without them installed. Unless your meter costs thousands of dollars, it isn't sensitive enough with a high enough sample rate to come close to catching the peak sound impulse.

Hi man, yeah I figured side exiting gas flow must have been done before but ive not found one myself. currently its exiting into 20mm dia tube that I took off my vacuum cleaner and because they are long, a large volume, its taking about 75% of the gas which would of traveled out the front.

Once ive confirmed exiting gases considerably quietens the silencing function then i will try to use those valves to exit gas into smaller chambers of pressurized gas. So once the shot pressure equalises in the exit chamber the valve will stop it returning into the silencer, hopefully But your correct in expecting the valve to limit gas flow.

[fishman]

You do know that this "gas exit right" and "gas exit left" concept is no different than a regular integral silencer, right? Its been done before 1000 times. The o rings are unique but I doubt they'll help a noticeable amount. Try it with and without them installed. Unless your meter costs thousands of dollars, it isn't sensitive enough with a high enough sample rate to come close to catching the peak sound impulse.

Hi man, yeah I figured side exiting gas flow must have been done before but ive not found one myself. currently its exiting into 20mm dia tube that I took off my vacuum cleaner and because they are long, a large volume, its taking about 75% of the gas which would of traveled out the front.

The mp5sd is the most well known integral design. It helps, but its only slightly better than regular silencer design.
Heres an example pic of the ported barrel. In my integral silencer: https://m.imgur.com/BU9mrZq?r
It leads to a ~100 cc chamber (6 in^3). The long tubes are entirely unnecessary.

[doubloon]

Seems like porting to bleed off an air rifle barrel would be a touchy endeavor.

IIRC some air rifle barrels are bored slightly smaller I.D. at the muzzle.

[ant3000]

This is the prototype I made and it shows the concept but it didnt silence as well as id expected but maybe there are some air gaps.

Its looking like a whole lot of work for hardly any return.

[a_canadian]

Air gaps? Seems you've got some sort of cloth tape perhaps, it's hard to guess from the picture just what is wrapped around that 90 degree hose juncture. You know they make 90 degree hose connector fittings?

But it seems unlikely that 'air gaps' are the source of your suppression efficiency problem. I see a common misapprehension here and in other NFA-related forums regarding just what is happening to sound inside a suppressor, and to be fair I've been guilty of sloppy language in this regard quite often. It's easy to be lazy when talking about gunshot noise suppression, to use euphemism in place of the language of science. Likely this relates to convenience in communication.

But so we're clear - diverting air flow (or combusting gas flow in a firearm) is not the primary cause of noise suppression. Rather it has to do with sound wavefront deflection. To help with visualization, think about shouting. A loud shout does involve some movement of air, of course, but really not very much air considering limited lung capacity and the potential for a sustained shout - think of a loud opera singer, for example. Hold a feather closely in front of the shouter's/singer's mouth and you'll see that the feather moves, but is not being blasted away. And yet a very loud shout or voice raised in song comes fairly close to airgun blast volume in acoustic intensity. Now, replace the feather with a 'sono tube' or length of rolled up carpet, perhaps a foot in diameter and 5 feet long, lined with foam points or even just egg crate foam of the sort used in garages to damp rock and roll band noise. The singer/shouter has their face right at the opening, the face almost filling the tube entrance. Is the tube filled with foam shapes stopping air flow? Of course not. What is happening is that sound pressure in the air between one end of the tube and the other is being interfered with. The expanding waves of sound energy are impacting the complex open cell foam shapes and being broken up into many reflected, smaller waves of sound, which then interfere with each other as they continue to crash into more foam shapes, interfere with each other, et cetera.

A gun suppressor can not be made in such a way. It would be blown apart on the first shot. A gun's report will be substantially diminished by firing through an appropriately scaled tube of such a kind (witness benchrest stations behind stacks of 20 or more tires, shooting through the middle of the holes), but such a setup is not practical for most types of shooting. So we use suppressors. Because of the gas flow, a suppressor's mechanical integrity must be scaled up to suit the intensity of the blast, with large calibre centrefire rifles taking the most material strength and engineering integrity, and air rifles requiring the least, but still significantly stronger. There are suppressors sold for air rifle use in the UK with only a few steel washers and a couple of hair curlers stuffed inside, but I've seen reports of airguns over 20fpe tearing these things apart in short order. Delrin or nylon cones or K baffles seem to be a minimum requirement, and when going over 30fpe even these can become damaged due to expanding air pressure.

So the structural requirements compensate for the high pressure gas expansion. But don't conflate that with diminishment of the report. The complexity and specific angles used in the baffles have much, much more to do with suppression of sound, breaking up sound waves and redirecting them, causing destructive interference of ever-greater complexity as the noise traverses the length of the baffle stack. Perhaps another analogy might help here. Think of two waves crashing into each other on the ocean. The energy of each is absorbed by the other. There may be a higher peak when they meet, but after that they are largely gone. Same with sound. Similar frequency sound waves striking each other cancel each other out. Active electronic hearing protection works in this manner, matching incoming noise with generated noise striking it and cancelling it.

Of course the variables involved are many, between projectile sizes and weights and velocities, the explosive noise of the powder charge involved (or sound wavefront of a PCP valve being slammed open momentarily and dumping air at 2,000psi or more very rapidly), and all the shapes and sizes and spacings one might use for baffle systems in various tube diameters and lengths. We arrive at our various suppressor designs via trial and error, intuition, the experience of those before us.

Your hose setup is interesting, as it adds to yours and to our collected knowledge. I think that it fails to accomplish your aim because it is reflecting noise in a fairly regular, uninterrupted series of bounces, down both lengths of tubing. It displaces the noise. And by simple destructive interference does something to diminish the noise, but is not designed in such a way as to maximize that interference. Adding baffles to your side tube might help more. Adding baffles to your primary tube certainly will. The volume of air being released is not directly relevant, only a secondary symptom of firing which must be compensated for in the structural design of whatever suppressor system you use. The acoustical problem is somewhat more difficult.

[ant3000]

Silent But Deady - Thanks so much for such an indepth reply and I basically dont disagree with anything. Just out of interest id like to address some of your points

1 - Sloppy language and poor references from me Im using knowledge ive got from Formula 1 aerodynamics and documentary channels, plus although im English my skills in this area are not good enough to pass end of school exams, so I apologise but I try my best to get the concepts over in my posts.

2 - How noise moves as opposed to blast from the barrel. You might have found my issue with noise from the 90 degree exit silencer. When I first stuffer the barrel into the garden hose and the shot with no pellet, it was silent and its apparent that the gas did not expand much unlike it does in the silencer tube so perhaps it did not get a chance to make the bang/pop noise??

3 - Materials made to build the silencer. Dealing with a PCP is great cos its gas is clean and cold. My K baffles are made from Nylon rods and the lathe cos its so much easier and the tools and quicker but I do like alloy as I can make accurate parts and they epoxy together fairly well or if necessary I can braize but that can cause to warping.







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The last test results
So I made a large sound dampening outer silencer to cover the end of the gun in an mostly air tight and 30mm thick closed cell foam for studios. Basically it did nothing. The gases were so slow they did not push out the outer foam circle, before it was duct taped in.

[ant3000]

Exiting gases before they create a bang noise

It seems to me that once that bang/blast noise has been produced its basically unstoppable within reason, slowing gases with baffles no longer helps at that stage.
The next tests is focussing on not giving the barrel gases a chance to expand which im thinking might be the cause of the bang. My theory is that the old silencers have a large enough area for the gases to expand and create the normal bang noise, the garden hose did not create any bang because it hardly expanded the gases but just slowed them as they flowed in one axis down the tube, this may be why it didnt create a bang noise but only another test will tell. My previous side exit silencer had one large side exit directed by baffles, but if the new one does not give the gases room to expand but rather has the same side exits with smaller 6-7mm tubes coming off the main barrel, then I can see how many are needed for the pressure to drop and then I can work on a baffled end silencer. If im lucky my mill will drill side exits at 45 degrees which will be an easier direction for the gases to exit.

So the nest silencer will have two parts, the first part in red is a series of 6-8mm ID hoses coming off the barrel ext, some test will tell how many are needed in order to relieve as much gas as i can to get close to all. then it should be easier to deal with the band noise as the pressure will be much lower.
Problems expected - if I keep the barrel ext the same diameter size as the barrel then the gases cant expand and create noise but we wont have baffles to slow or redirect gases away from the bullet path and into the tubes. Theory is to get the barrel ext made with ID 6mm but if that doesnt slow the gases enough to bleed off then I will drill them out to 8mm or so and add a few small baffles to aid redirecting gases. Then I will be able to see if it that bleeds off a high enough percentage of gas and reduces the speed of the remaining gases so it doesnt make a loud band noise.

Im going to bleed the gas off into a water bottle so that I can measure how much air is coming out.