Sound Suppressors: Hush, Puff, Shhh
By Paul Evancoe
Hollywood movies depict sound suppressors as “silencing” the report of a handgun or rifle to an almost inaudible “puff - puff’” sound. Most have no idea why or how suppressors work, why some designs work better than others, what role the choice of ammunition plays, or other critical operating dynamics that involve gun design and the unavoidable laws of physics.
Quite simply, a sound suppressor works by slowing the sonic (traveling above the speed of sound) swirling high velocity gases generated by the propellant’s burn to sub-sonic speeds as those gases leave the suppressor’s muzzle end. It took a modern scientific understanding of hydrodynamics to recognize that sound suppression pivoted upon two variables – reducing bullet velocity to less than the speed of sound (1,126 feet per second at sea level) and slowing the sonic high velocity gases generated by propellant combustion to sub-sonic velocity. This slowing of the swirling sonic velocity gases is scientifically described as disrupting the hydrodynamic axially rotating pipe flow – well; now you know.
The first condition for suppressing the firing sound (report) made by any firearm is to use sub-sonic ammunition (or porting the barrel itself to reduce the speed by reducing volume of propellant gases). Sub-sonic ammo leaves the muzzle below the speed of sound, with a velocity less than 1,126 feet per second. This is most often accomplished by “down-loading” the ammunition by either loading it with a slow burning sub-sonic propellant (smokeless powder), or using fast burning propellant ammunition in conjunction with a short-barreled gun thereby reducing propellant burn time which subsequently reduces bullet velocity.
The use of sonic rounds, especially in long guns like sniper rifles, that snap, crack and pop as they break the sound barrier in flight downrange is commonplace. The advantage of using a suppressor in conjunction with high velocity ammunition is that locating the shooter’s exact firing position is made difficult and it further reduces the requirement for ear protection by the shooter, allowing better situational awareness. Secondly, there is little loss of muzzle velocity or impact energy beyond that which would be experienced without the suppressor. Unless the gun has a floating barrel, accuracy usually isn’t affected excepting predicable shifts in group point of impact.
Using a suppressor on a floating barrel involves several factors that are many times not considered. A floating barrel improves a long gun’s accuracy. Suppressors add weight to the barrel’s muzzle-end. Adding weight to the muzzle-end of a floating barrel will change barrel harmonics thereby reducing accuracy. This can somewhat be overcome with a heavy target (“bull”) barrel that is structurally robust and not as susceptible to harmonic effects. However, the added weight of a target barrel and a sound suppressor isn’t always desirable in a tactical environment where mobility and man-carried portability is a necessity.
Even with a suppressor and sub-sonic ammunition, there are several steps that can be taken to further reduce the firing signature. The first is simple. For maximum suppressor efficiency, the gun needs to be “locked up.” This means a manually operated bolt gun with a suppressor offers less of a sound signature than a semi-automatic gun of identical barrel length with the same suppressor shooting the same ammunition. Here’s why. Upon discharge, a semi-automatic bolt strokes rearward to eject the spent brass. The bolt’s backstroke allows some of the high velocity propellant gases generated upon firing to escape the breech and that makes sound. Additionally, the mechanical operation of the retracting and advancing bolt makes a mechanical sound that, in most firearms, both rifle and pistol, approximates the same noise made when manually jacking the bolt/slide back and forth when chambering or unloading the gun. It’s noisy and can be heard.
The mechanical operation (and associated mechanical noise) of a semi-automatic (or automatic) rifle or pistol can be eliminated by the installation of a lock-up lever that prevents the bolt or slide from cycling when the gun is fired. This forces all the gases of combustion to exit from the muzzle end of the gun through the sound suppressor. The down side of this is that it relegates the firearm to single fire requiring the shooter to unlock the slide/bolt, and manually eject the spent brass and reload (manually cycle) the gun. Clearly, where optimal sound suppression is necessary and only one shot is required to get the job done, this option is desirable. Where multiple shots may be necessary, the use of a slide/bolt lock is suicidal.
Incidentally, using a suppressor with a revolver is only done in Hollywood. If you’ve ever seen a revolver fired at night you will have observed the flames emitted between the cylinder and the barrel. No matter how well it’s fitted, the gap between the cylinder and the barrel always allows gas to escape and that means it “bleeds” noise. Don’t waste money putting a suppressor on a revolver.
As mentioned previously, along with other factors, the selection of ammunition is important. The propellant’s burn velocity, amount of propellant, bullet weight and barrel length (providing enough length for a full propellant burn), are directly proportional to bullet (projectile) muzzle velocity and effectiveness on target impact. When using a suppressor, the choice of ammunition must be carefully considered. Obviously, ammunition that offers the highest energy on target impact with the lowest sound signature is most desirable. Sub-sonic ammunition is commercially available for many of the more commonly used calibers. In fact, most pistol ammunition is already sub-sonic, or very close. A bullet velocity of less than 1,200 FPS, even though it is not advertised as sub-sonic, will work just fine in conjunction with most short barrel pistols and sub-guns equipped with suppressors.
One of the most common problems encountered when using downloaded sub-sonic ammunition is that a self-loading (semi-automatic or automatic) firearm will many times not function reliably. This most often results from the bolt/slide not retracting backward with enough force to reliably eject the spent brass, or not stroking rearward far enough to pick up the next round from the magazine, chamber it and return to battery. Feed malfunctions and jams are commonplace unless the recoil spring/buffer mechanism is modified and tuned to properly operate the bolt/slide under reduced recoil (lower ammunition operating pressure). In most cases, this means that a gun modified to function with sub-sonic ammunition can then no longer safely shoot standard high velocity ammunition without damage to its operating system or the shooter. That’s the reason most guns that are modified to shoot sub-sonic ammunition are dedicated for that specific purpose.
The exception to this rule was perfected in the 1970s with the advent of the Electrical Discharge Machining (EDM) process. Using an EDM, a small gas-venting channel (groove) is cut inside the gun’s bore from a point behind the bullet of a chambered round to a measured point in front of it. When the gun is fired this channel vents high-pressure gas around the bullet slowing its travel down the bore to sub-sonic velocity. This permanent gun barrel adaptation permits the use of regular high velocity ammunition in sub-sonic suppressed applications.
So where did it all start? The evolution of sound suppressors began around the time smokeless propellant became the mainstay early in the 20th century. Wrapping a pistol in a towel or blanket reduced its sound signature. That worked fine for targets at arm’s length but not for distance. Around 1902, American inventor Hiram Percy Maxim, developed, patented and sold the first commercially available suppressors under the trademark, “Maxim Silencer.” Today there are 3 basic suppressor designs with internals that employ either screens, wipes or baffles (there are also some very clever hybrids) that are inserted inside a can-like outer housing of sorts and the can is attached to the muzzle end of the firearm. Let’s walk through these designs as they chronologically evolved and better understand them.
Screen-type Design. A prime example of a screen-type suppressor was used on WW-II vintage M3A1 .45ACP sub-machine guns (grease guns) carried by paratroopers. Some of these guns were specially fitted with screen-type sound suppressors. They worked by passing the high velocity gases escaping through a specially perforated barrel through dozens of screens inside the suppressor’s can (body). This slowed the gasses, all but stopped the Coriolis (natural tendency for gases to spin around an axis) and quietly vented the overpressure to
atmospheric pressure.
A screen-type suppressor is perhaps one of the most elegant yet least sophisticated and cheapest designs to build and maintain. The suppressor body (“can”) is made from a piece of 2 1/2 inch diameter (approx.) seamless steel pipe about 12 inches long. It has a threaded removable faceplate end cap with a centered .45 inch bullet exit hole. The threads are precision optical-type 60 degree included threads. The faceplate’s outside edge has a straight knurl to improve the finger grip when tightening and untightening it to the can. The other end of the can has a permanently welded end cap that is threaded to mate with the shoulder threads on the gun barrel’s breech end. When it’s screwed onto the gun, the perforated gun barrel extends completely inside the can tightening flush with the gun’s breech.
The suppressor’s internals are made from common 80 to 120 mesh metal window screen punched out to fit the inside diameter of the 2 1/2 inch can. A hole is also punched exactly into the center of each piece large enough to slide over the perforated gun barrel. When enough of these screens are punched out (it literally takes dozens) to completely fill the inside of the can, layered one directly against the other, they are loaded into the can by first stacking them onto a center diameter wooden or metal dowel. Then, using the dowel, they are inserted inside the can as a whole element. The can’s threaded end cap is secured finger tight and the can is screwed onto the gun’s muzzle. It’s now ready to shoot.
After hundreds of rounds fired the screens tend to clog up with carbon and unburned powder residue. To clean the suppressor, the can is removed from the gun. The threaded end cap is removed. The dowel is reinserted into the center hole of the screens. The suppressor is up ended and the screens are all removed at once on the dowel. The screens and dowel are submerged in gun cleaning solvent. When the dirt is soft the screens are either individually brushed (field cleaning) or compressed air is used to blow out the dirt. The screens, using the dowel, are reinserted into the can, the end cap is replaced and the suppressor is ready for use again.
The advantage of this suppressor design is that it was cheap, low tech and it worked reliably. The downside was its weight from all the metal screens and robust steel housing. It also rusted if not maintained and some screen replacement was regularly necessary after sustained automatic fire because the heat of combustion and high velocity gases deteriorated the fragile screen material.
Wipe design. Of the 3 basic designs previously mentioned, the use of wipes followed the screen design. Wipes began appearing in suppressor design in the mid 1960s during the Vietnam War. A prime example of the operational use of a wipe suppressor was one used by U.S. Navy SEALs nicknamed the “Hush Puppy.” SEALs primarily operated at night. Sneaking into or around the small rice paddy villages and hamlets always meant risking compromise from barking dogs. The answer was a modified Smith and Wesson Model 39 with a Hush Puppy sound suppressor. A lot of dogs fell to this 9mm sub-sonic sound
suppressed pistol.
The wipe design is perhaps the quietest and smallest of all suppressor designs but it has the least lifespan and requires the most maintenance. It works by using a series of spaced rubber-like baffles called wipes, that the bullet passes through on its travel through the suppressor. In this design the bullet actually makes contact with the wipes that flex open and close as the bullet passes through them. This traps the propellant gasses inside the baffle chambers which results in a near complete sound suppression of the subsonic round to something equaling a pen tap on a desktop in a quiet room. Obviously the SEAL’s Model 39 was modified with a slide locking lever so it wouldn’t back stroke when fired. This dedicated single shot application proved wildly effective to quiet dogs as well as enemy combatants. Another example of the smallest wipe design used by the SEALs was a single fire suppressed gun used for assassination that was made to look like a ballpoint pen. The one shot gun fired a .22 cal. CB cap when the pen’s ballpoint extension button was pushed down. Its report was negligible and when fired at the base of the skull it was lethal.
In general, here’s how Hush Puppy wipe design works. The rubber wipes are punched out of 1/4 inch thick butyl rubber sheet material with an outside diameter that fits the inside diameter of the suppressor’s can. Each wipe has a small 1/8 inch hole in its center. Each rubber wipe is then carefully sliced into quarters (or sometimes sixths) that radiate from the center hole. The quartering cuts stop about ¼ inch from the wipe’s outer diameter so it remains intact looking much like a cut pizza with an uncut outer crust ring.
The rubber wipes are then inserted into the suppressor can with metal spacer rings separating each wipe. The spacer rings serve two purposes. They create small gas-catching chambers between each wipe and provide additional internal structural integrity that prevents the wipes from being torn away as the bullet passes through them. Because of the efficiency of the wipe design it can be made very small (scaled down to a particular application like the pen gun mentioned previously) and thus, very light. The down side of a wipe design is the fact that the bullet wears the wipes every time it passes through them. This translates to a suppressor that gets louder every time it’s fired. Most wipe designs only maintain their suppression integrity for about the first 10 shots. Then the wipes must be replaced with new wipes. Anything beyond 10 shots results in a sequential increase in the firing noise. At approximately 20 rounds there is essentially no longer any suppression left because the wipes have been worn away.
Baffle design. This design is most prevalent today with numerous types of exotic baffle shapes that range from cones to flat washer-like shapes along with the use of common to exotic metal to composite construction materials. In general terms, most all baffle designs work the same. They slow the sonic gases resulting from propellant combustion to sub-sonic speeds using baffled chambers and vent the gases harmlessly to the atmosphere. Baffle designs are also the most environmentally rugged, have the longest life expectancy and are the easiest to maintain. In short, they work and they last thousands of rounds.
Early baffle designs used little more than washer-like steel rings welded inside a pipe-like housing. Similar to the can used for screen-type and wipe designs, the baffled suppressor uses a can with a bottom end cap that mounts to the gun’s muzzle end and a front cap (usually threaded and removable for cleaning maintenance) on its bullet exit end. Beyond the visible can-like exterior, all comparison ends.
Around the early 1970s an out-of-the-box-thinking gun enthusiast and machinist by the name of Mickey Finn, began experimenting with the physics involved in suppressor technology. Finn was on a one man quest to increase the efficiency and shrink the size and weight of sound suppressors. Following in-depth trial and error experimentation, he recognized that the high velocity sonic gases resulting from propellant combustion could be radically slowed by creating baffles with angled machined cuts that created reverse flow venturi-like jets that redirected the high pressure gases in opposition to one another within each baffle chamber. He further realized that equal spacing of the baffle chambers was not optimum. Finn adjusted chamber spacing (and thus, volume) to account for the progressively slowing gasses thereby providing for optimal suppressor performance.
Finn began trial and error limited production of his uniquely designed suppressors and enjoyed radical success with the SEALs becoming his best customer. But the SEALs had a unique requirement that no one had previously met. After being submerged, the SEALs needed the capability to fire a flooded suppressor-equipped gun when taken directly out of the water without any drain time. Finn set to work on the suppressor technology while the Naval Surface Weapons Center’s China Lake and Crane Indiana laboratories modified the SEALs assault rifles and machineguns at critical points with drain holes and self-lubricious coatings.
The SEALs had previously tested Finn’s suppressors by firing them when only partially drained. They found, curiously, the suppressors were actually quieter when they contained some water, but why? A puff of steam-like vapor followed each shot until there was no water left inside the suppressor. Without the water the suppressor was a bit louder. After some hydrodynamic soul searching they arrived at an answer that forever changed the understanding of suppressor dynamics.
Finn’s opposing gas check baffle design efficiently reduced the size and weight of the suppressor. Adding additional gaseous particulate material (like water particles) to the mix slowed the sonic gases even more. The result was a hybrid suppressor that was called the “Greaser.” The Greaser while still using a reduced version of the ultra-sophisticated opposing gas check baffle design was altered to have its base end stuffed with several thumb loads of commonly available bearing grease (but any grease works) prior to mounting it on the gun. The result was additional sound suppression.
Here’s how it works. When the gun is fired the rapidly expanding high pressure sonic gases enter the bottom end of the suppressor where the grease is located. The gases instantly atomize some of the grease. The atomized grease thickens the suppressor’s internal atmosphere with a heavier (more viscous) medium that slows the gases more efficiently. This essentially allows a smaller suppressor to perform at about the same sound suppression level as a larger unit.
The down side of the Greaser is that it emits puffs of blue vapor/smoke until all the grease is expended. That notwithstanding, the greaser can be used like any other suppressor until the grease can be replaced. Regardless, the science behind this revolutionary concept in suppressor technology has been largely forgotten in today’s baffle designs. That said, many of today’s suppressor designs can accommodate the addition of grease or other more modern material, and operate quieter as a result. Most manufacturers either don’t know about this option, or won’t advertise it as an alternative because of potential maintenance/warranty issues.
A quick aside: How do you know when to clean a suppressor? The answer is simple. Follow the manufacturer’s suggested planned maintenance schedule. Or, as a field use rule of thumb, when a suppressor begins to get noticeably louder and louder - that’s an audible signal that the suppressor needs cleaning and/or maintenance.
Today’s baffle design offerings are numerous and their levels of design sophistication vary widely. Some work far better than others. Some look outwardly awesome but have inefficient internal designs. Some look unimpressive outwardly but work exceedingly well. A suppressor’s retail price doesn’t necessarily mean that you’re getting what you’re paying for either on the high or low end of the spectrum. After reading this article, you should now be better able to do some intelligent evaluation regarding suppressor design, maintainability and life expectancy. Choose wisely.
This article first appeared in Small Arms Review V20N3 (April 2016) |
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