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).
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.
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.
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).
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.
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.
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.
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.
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.