Let’s chat about suppressor design

DISCLAIMER: 
I’m assuming that if you are reading this, you are an intelligent adult who is aware of what laws govern sound suppressors in your country. In my country (the good ol’ U.S. of A), it is illegal to manufacture a suppressor without seeking approval and obtaining a tax stamp from the Bureau of Alcohol, Tobacco, Firearms, and Explosives (BATFE) via a Form 1, or via obtaining a Class 2 Federal Firearms License and paying the appropriate Special Occupational Tax (SOT). Don’t be stupid. Follow all laws and play by the rules.

If you haven’t already guessed from my post about making a .300 Blackout piston gun, I’ve been super fascinated by sound suppressors over the past two years. It’s hard to believe, but almost one year ago I built my very first sound suppressor out of a Maglite flashlight (via ATF Form 1). Since then, I’ve procured a SilencerCo Harvester 30 and made another Form 1 suppressor. One of the things that truly astounds me about suppressors is the technology and engineering involved in making them work effectively. Now that I’m fresh off of my second suppressor build, I thought it would be a great time to go over some ideas I’ve had about sound suppressor design.

One of the tasks you must accomplish prior to filing an ATF Form 1 is to draw up a diagram that you will submit with the form. I truly enjoyed this part of the process because it pushed me to look around and get inspired by all of the different designs out there used by the big name manufacturers. One bit of inspiration came when I stumbled across a post by the Truth About Guns blog that has photographs of several big name manufacturer’s suppressors cut in half, exposing their baffle designs. Check it out.

It sounds weird, but I could look at suppressor cutaways just about all day and it would just pump me up about wanting to design a baffle stack…

Oh, sorry… got lost in them again…

Since I’ve been staring at those photographs for far longer than I will ever admit, I’ve begun to postulate about what design features might make suppressors quiet and efficient. Before we delve into this, however, let’s go over and define some nomenclature:

Suppressor Body/Main Tube – this is the exterior portion or tube portion of the suppressor.
Suppressor Baffles – These are the structures within the suppressor that disrupt the flow of propellant gases as they exit the barrel and form chambers within the suppressor which allow those gases to expand to a lower pressure. They come in all different shapes, but we will be using conical ‘M’ baffles in this discussion.
Primary Blast Baffle/Chamber – This is the very first baffle in the baffle stack. It creates a chamber that will take the major brunt and abusive force of the propellant gases right as they enter the suppressor. I like to reinforce the primary blast chamber using thin steel tubing (it will also act as a spacer for adjusting volume).

Basic Baffle Stack
Basic 8-Baffle Stack

Thread Attachment – I wasn’t sure what to call this one, but this is the back end of the suppressor that affixes to the barrel. This can either be directly threaded to the barrel, or somehow affixed to a threaded muzzle device.
End Cap – This is the front end of the suppressor that holds the suppressor together and is the last thing the bullet will travel through.

General Design
There is a lot of hearsay on the internet about proper suppressor design in conjunction with intended use. What does that mean exactly? Well, the word on the web is that for high pressure, high velocity rounds, you need more volume and fewer baffles. For lower pressure, low velocity rounds, you need less volume and more baffles. Why? Because reasons (obviously). Let’s look at one more image I shamelessly stole from the internet that seems to demonstrate the cartridge max pressure to suppressor volume hypothesis.

suppressor-x-ray

As you can see, all of the centerfire rifle suppressors have large internal volumes and fewer baffles…. Hmmmmmmmm…

If there is any single truth I do know about suppressor physics, it is this: It’s effing complicated. If you took a look at the suppressor cutaways, you probably saw that there are many different designs out there and some of them seem to demonstrate the web’s pressure/volume theory. However, what I can conclude from my ‘research’ is that there doesn’t seem to be a consensus in the suppressor industry as to what design elements make one sound suppressor quieter than others. Be that as it may, there does seem to be one universal principle: For a suppressor to be effective, it needs baffles to disrupt and slow down the flow of gases exiting the barrel, and it needs internal volume for those gases to expand and pressure to dissipate. There doesn’t seem to be much compromise in this regard; you need both to be quiet. We’ll call this the baffle/volume principle.

Let’s take a look two examples from SilencerCo (I do not own the picture below; I selfishly stole it from a Pintrest post).

specwarsaker762

The image above is a cutaway look at a Specwar 762 (top) and a Saker 762 (bottom) from SilencerCo. Both of these sound suppressors are rated all the way up to .300 Remington Ultra Magnum (roughly 65,000 PSI max pressure). As you can see in both suppressors, there is a larger volume of space between the muzzle and the primary blast baffle than between all of the following baffles in the baffle stack. That is because the majority of expansion from the propellant gases will take place just as they leave the muzzle. Now, it is good to note that the Specwar has much more volume for expansion and one more baffle than the Saker. According to SilencerCo’s website, the Specwar is also quieter…

Now, SilencerCo is able to make a short .300 RUM suppressor like the Saker 762 with such little volume by using incredibly strong, and equally as expensive Stellite for baffling that is fully welded to a stainless steel inner tube, making a solid brick-shithouse. The average consumer won’t have the machinery or money to build such a tenacious critter like the Saker 762, so if the common man (or woman; I don’t stereotype) decides to build a suppressor, they need to have proper internal volume to make a safe and effective suppressor. Just remember this principle: The higher the pressure, the more volume is needed for those gases to safely expand and slow down. If you don’t accommodate the needed expansion volume properly, you might make an unsafe suppressor or at least one that’s not as quiet as it could be.

I digress…

In regards to the baffle/volume principle, the most effective sound suppressor ever made would technically have a considerable amount of volume and an ungodly number of baffles. Unfortunately, unless you want to carry around a retrofitted water heater filled with massive funnels to suppress your gun, you must find a compromise that will fit the suppressor’s size and intended use. Because of this compromise, there will be a battle for the suppressor’s internal volume as it is consumed by baffle material. For each baffle you put into the suppressor, that is volume being displaced by baffle material. So, a general rule of thumb to consider is: More baffles = less volume | Less baffles = more volume.

Let’s say we want to build a suppressor that is 8″ long in overall length, with a main tube interior diameter of 1.35″. In this scenario, the end caps will take up roughly  .25″ on either end of the suppressor, giving us a main tube of about 7.5″ in length. The end caps will also thread into the main tube about 0.5″ on both ends, giving you roughly 6.5″ of space inside of the main tube.

emptytubevolume

Ideally, we want to use as many baffles as possible whilst retaining as much of the original volume as possible. Now, for you math geeks out there, 1.35″ inner diameter with 6.5″ of length = 9.3 in³ of volume. If you were able to find baffle material, like a 1.35″ diameter, 0.5″ tall steel cup with 0.125″ thick (1/16″) walls, or about the rough dimensions of an appropriate freeze plug, it would take up about 0.42 in³ per baffle. Let’s go further by saying that the spacer tube you plan to use is roughly 1.35″ outer diameter with 0.125″ thick walls, you can subtract 0.32 in³ for every 0.5″ of spacer tube used.

Let’s start with a high-pressure cartridge setup as a benchmark (for supersonic .308 Winchester or 5.56 NATO). We will have a 2″ primary blast chamber, 8 baffles, and a 0.5″ spacer between the primary blast baffle and the rest of the baffle stack. This gives you a setup similar to the diagram seen in the nomenclature section above. With this setup, roughly 47% of the original volume is retained and roughly 53% is taken up by spacer and baffle materials. Albeit basic, this is a solid high-pressure cartridge setup that might not be the quietest can in the world, but is pretty damn effective.

hivelocitytubevolume
High Pressure/Velocity Rifle Suppressor

From here, you can modify the design to be optimized for lower pressure cartridges, like subsonic rifle, handgun, or rimfire cartridges (which require less volume). Let’s cut the spacer tube 1″ shorter and add two more baffles.

full can diagram low pressure
Low Pressure/Velocity Pistol Caliber or Subsonic Rifle Suppressor

As you can see, there is less internal volume, but since you will be shooting lower pressure cartridges, it will need less internal volume for expansion. This would make an exceptional handgun caliber or rimfire suppressor, and an excellent subsonic rifle suppressor.

This is by no means the only way to accomplish effective suppression, but just one guy’s opinion. I am genuinely fascinated by the engineering and physics behind sound suppressors, and I’m always interested in learning something new. If you have any insight or knowledge on this topic, please feel free to comment and share what you know.

Advertisements

18 thoughts on “Let’s chat about suppressor design

  1. This is a great article, thank you for posting it.
    Now a theory question:
    Would a shorter barrel require a bigger expansion chamber due to the excess gasses stemming from the less optimal burn? Or to put it another way would you want a higher velocity suppressor design for a shorter barrel?

    Like

    1. For a shorter barrel, yes, more expansion space will be necessary to make it quieter because much more of the powder will be burnt outside of the barrel. The big issue with short barrels, however, is baffle and tube erosion. Much more of the explosion from the powder charge is burning outside of the barrel, which means much more concussion is happening in the primary blast chamber. To combat this, you’d need to have a fully encapsulated primary blast baffle of a ridiculously hard material, like inconel or stellite, much like SilencerCo’s Hoplon design. This will keep down baffle erosion substantially, but will definitely increase weight.

      Like

    2. Have you ever thought about a secondary layer around the baffles with the screen and insulation system like the old glass pack muffler design? Is lessening the pressure within each chamber between baffles create more, or less suppression?

      Like

      1. A long time ago, yes. For the most part, the less pressure there is between baffles, the quieter it will be. I thought about some sort of insulating wrap to go around the internal baffle structure as a way to slow gases through dispersion. However, the big issue I run into is volume. If I try to add stuff inside of the tube, i’ll lower the overall volume and increase pressure. I’d need a much larger tube to achieve better performance than the basic baffle design I’ve made so far.

        I remember reading about some company that made a glasspack like sound suppressor for .22 LR. Apparently it was really quiet for about 10 shots, but was worthless after those first 10 shots.

        Actually, it brings back a memory that led me toward trying to make such a contraption. I remember encountering a sound absorbing wall when I worked for the University I went to, and the wall really threw me off. Any time one of your ears faced the wall, it was like something was plugging your ear. The wall itself was made of a lattice-like layered plaster that was super porous, so sound couldn’t reflect off of it. The closest thing I can find to the way it looked is this stuff here called wood wool. That gave me the thought of making a .22 suppressor with a layered fire-resistant lattice that made up the innards of the tube. I was never able to find any fire resistant material easy enough to work with to make the thing, but I bet it would make a fantastic limited-use suppressor.

        Like

    1. >*First, please make sure you file all of the proper paperwork and have the approved BATFE form 1 and tax stamp IN HAND before even starting construction*<
      The high pressure/high velocity suppressor diagram I've got on this blog post will work fine for 5.56. The key is to use a very hard material for your Primary blast chamber and primary blast baffle. That should help keep baffle erosion down substantially. You can repurpose Stainless Steel D-Cell Dry Storage Cups for a primary blast baffle. The more volume in the primary blast baffle, the safer the can will be. You can form steel freeze plugs into M baffles, and that can finish the rest of the baffle stack. Also, The more concentric you can make the baffle bore, the tighter you can make the bore diameter, and the quieter it can be. If you have access to a lathe, getting bore concentricity will be simple.

      I'd try to help you through here, but there are lots of forums and tutorials easily found online that would be much more helpful than I could ever be. Good luck!

      Like

  2. Very good post. Supressors are a science and engineering treasure trove, love them. You forgot to mention something very important. Vacuum pressure causes a good amount of air to continue to follow the bullet past the first, second, third (etc) baffles. This is why SWR (SilencerCo) has a nice, albeit smaller, chamber at the end of the baffle sections. It is also why they have two mid sized chambers after the first blast chamber/baffle. It allows that air/propellant mixture following that boat tail bullet to have a greater chance of being caught before exiting the tube. Those extra volumes in those places allow propellant to finish burning/lowering pressure in that last section.

    There is also another characteristic on the SWR suppressors that you should point out, look at the shelf after the radial curve of the baffle. They are nice 90 degree angled corners and on the first sets of baffles the radial ramp goes straight to them. This forces the pressurized gas to slam right into the corner and fold in on itself, into the other 90 degree shelf corner and back into the void in the chamber. The baffle angle changes as it goes down and then causes a swirl to reverse flow forcing the gases back into the first baffles. This is a good design and optimizes the energy dispersement to counter itself as much as possible before leaving. Which is ultimately the name of the game.

    This is something overlooked by some form 1 designs that use M or B baffles. Those rounded sections (where your arrows are pointing on your diagram) cause gases to swirl back into the projectile path and it continues to move forward and eventually out of the suppressor before it has lowered its pressure. Baffle design makes a huge difference and can lower the volume necessary.

    Like

    1. Yessssss! This is why if find the science so fascinating. Even the subtlest nuance in the suppressor design can have huge impacts on performance. Had I machinery to make proper baffles, I’d probably have designed it with a much more aggressive taper on the baffle, and a flatter wall at the front. Good info, man!

      Like

  3. It is NOT necessary to furnish a diagram of a silencer/suppressor with a Form 1.
    Block 4(a) says:
    Name and Location of Original Manufacturer of Firearm
    (Receiver) (If prototype, furnish plans and specifications) For a RECEIVER FIREARM, not a silence/suppressor.
    For a Form 1 silencer you put Your Name, Street Address, City, State and ZIP Code.
    Block 4(b) will say SILENCER
    Block 4(c) Caliber or Gauge (you put like 5.56mm., or .223 or 30 Caliber or whatever)
    Length
    Block 4(e) is N/A
    Block 4(f) Overall (longest length you expect it to be, if ends up shorter is no big deal)
    Block 4(h) Your Name, City, State (2 letter Postal Code is acceptable), Model (optional), Serial and Caliber.

    I have 2 Approved Form 1s for a Silencer/Suppressor and did not submit drawings.

    Like

  4. I have built two 300 Blackout rifles for hog & coyote hunting in Alabama. I have thermal on one and IR on the other. I want to construct two licensed suppressors. My present intent is to have a primary blast chamber approx. 6 inches which overlaps barrell, and stacked freeze plugs for about 5 inches of extension past 16 ” barrell. Thinking of titanium outer tube and modular assembly. Not threaded ends. I am an engineer/lawyer and can fabricate about anything. I would truly appreciate your commentary on my intended design.

    Like

    1. What you are describing sure sounds a lot like a reflex suppressor. I love those designs! I originally had an idea of doing something similar, but having the affixment point at the gas block with an internal index on a muzzle brake to keep the suppressor bore concentric to the barrel bore. The muzzle brake would also direct primary gases rearward to expand into the reflexed portion of the can. Wait… Sorry, I have to stop for a second to calm down…

      I think this is probably one of the sexiest designs that any suppressor could be. Since I’m not a machinist, nor have I ever had access to one, I was never able to make a gas block with forward facing threads to make one of these. My god, I want one though… I bet it would be super effective and look like pure, unfiltered sexy.

      Now, making it without threads has its set of challenges. I imagine if you can get a proper interface, like an H&K 3-lug or similar, you might be able to get a proper seal. What do you have in mind?

      Like

      1. Thanks for naming my concept as reflex. Found some. That’s it! I have a copper tube which tightly fits outside of bbl at 0.75 in.dia. It will be crimped to pinch behind flash hider and cover inside of reflex. Copper will silver solider to a forward-facing steel freeze plug bored to fit over bbl. I am going to attempt to attach outer tube to rear plug with multiple external screws. Another set of screws around end of flash hider will also be threaded into a freeze plug which is welded to the end of flash hider.Baffle stack will be secured via steel rods welded to keep them spaced and into alignment. This assembly will be easy to clean or tweak. Baffle Assyrian mounts when flash hider is tightened. When outer tube is installed over baffle assembly and both sets of screws installed, whole suppressor is reliably bore-centric. My OD of tube is close to the OD of quad rail and fits so close to end of quad that it allows a sliding pin to keep tube and flash hider from turning from installed position and loosening from repeated use. Since I have a special night-hunting purpose of use, I should benefit from easy take-down and cleaning between hunting excursions. After all, my battery’s will have to be removed and charged after each hunt. Suppressor service can be just another item on my prep list.

        Liked by 1 person

      2. Sounds like it’s going to be heavy, but pretty legit. I’d be interested to see how that copper tubing will do over time. I imagine you might see some permanent expansion, and potentially flowing, if it gets too hot. The worst that might happen, which isn’t bad, is that it squeezes tighter against the barrel. I wish you the best of luck and good harvest!

        Like

  5. Good discusion on baffle angle of incidence above. Thanks.
    You have probably played with the volume ratios (ref. 47/53 %), have you thought about drilling small holes/slots radially in first series of baffles to increase the volume for expansion?

    Like

    1. I did think about it. What worried me about doing so was disassembly and maintenance. My freeze plugs were widest at the bottom and had a slight inward angle up to the shoulder. This made the base of the freeze plug the effective seal as a baffle, and if you cut holes radially, you wind up losing the seal. Now, having a perfect seal in the first few baffles isn’t really necessary for function or performance, but it sure makes it easier to take apart and clean. If you have radial holes or cuts in the baffle, that’s room for carbon fouling to cake into the cuts and bond to the inner tube of the can. That could potentially lock the baffles into place, and that is no bueno. I imagine it would increase suppressor effectiveness by freeing up volume, but I’ve already had to hammer out my baffles a couple of times and I can’t imagine what it would be like having them get seized up in the can from carbon buildup. I might wind up destroying my suppressor trying to get them out :(. If you have a little wiggle room in your suppressor, it might be worth trying. Mine is kind of tight, so I probably won’t.

      Like

  6. I have a FP form 1 suppressor ready to.assemble. I am still unsure about adding spacers, how many and where in the stack. Also, should the blast chamber and spacers fit as snugly as the freeze plugs. Any assistance would be greatly appreciated. I have form 4 suppressors, but this is my first build.

    Like

    1. Just like the rest of the baffles, the primary blast chamber doesn’t have to be friction fit, but the more snug, the better, and concentricity is key. If you need to shim, I’ve used parchment paper before to shim around the spacer tube that makes up my primary blast baffle. It’s not a permanent fix, but the parchment paper holds up to heat exceptionally well. After shooting for a bit, the can will fill up with carbon and really tighten up all of the little gaps, so don’t fret too much.

      As for spacers, like I discussed in the article, it will depend on the dimensions of your suppressor and your intended application. I am no expert, but I can provide some opinion on the topic. If you are planning on using the suppressor for high pressure applications, you may need to add an extra expansion chamber, like a secondary blast baffle, for expansion volume. This would involve adding a section of spacer tube between the primary blast baffle and the rest of the baffle stack. If it’s a lower pressure application, you may not need any spacer tube at all. After the primary blast chamber and primary blast baffle, just stack the rest of the baffles until you run out of room. If you are having issues with your internals moving around inside of your can while it’s assembled, then I’d suggest adding a spacer of some kind to the muzzle end of the suppressor to tighten it up. I wound up welding a washer to the last baffle to tighten up about a 1/16″ bit of play I had in my suppressor. Your solution might vary

      Once again, this is just opinion. These are the two setups that I’ve used and both work well. Also, be sure to take a look at cutaways and x-rays of factory suppressors for inspiration. Get informed, get inspired, and get creative.

      Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s