Most shooters have been to indoor and outdoor ranges for an outing at one time or another. For those who’ve shot at an indoor range, there’s been a time or two when the shooter next to us fired off a gun and we felt that concussive blast. When it happens close to me, my first thought is normally, “What the heck was that?!”
This experience is what is commonly referred to as “overpressure.” Overpressure is sometimes defined as a cartridge exceeding the specified pressure for the firearm. In this context, overpressure is the external pressure change that occurs as a projectile exits the muzzle. Overpressure depends on a variety of conditions including barrel length, caliber, and how confined the environment is. For most of us, this is a very important consideration that should be accounted for in your defense plan, personally or professionally. Unfortunately, few of us know what affects overpressure or why it matters.
Why Overpressure Matters
If you’ve ever shot a firearm in an enclosed place, it’s an experience. In an earlier article, I demonstrated how muzzle flash is a factor of caliber, barrel length, and ammunition type. During photography for that article, I set up my office to shoot through an open door with low lighting to capture the muzzle flash. Unfortunately, shooting inside that enclosed space made for an interesting experience. The concussion from the muzzle blast on some of the firearms was impressive and somewhat uncomfortable, even with hearing protection.
When staging a firearm and selecting ammunition for home defense, there are a variety of elements to consider: overpenetration considerations for selected ammunition, accessibility of that firearm to responsible adults, restricting access of that firearm to children, and numerous other elements to keep in mind. For a lot of us, overpressure is something we don’t consider or factor into our personal defense plan.
Overpressure occurs by the expansion of gasses as a projectile exits a barrel. This is partly the source of hearing damage from a firearm discharge. Bullets that exceed the sound barrier generate a sonic crack as they exit the barrel. In addition, the expansion of gasses causes noise and a pressure change in the immediate area. Sudden changes in pressure can cause temporary or permanent hearing damage if adequate hearing protection isn’t used. Suppressors help mitigate this by slowing the expansion of gasses exiting the muzzle. As a result, the potential for hearing damage is reduced. Overpressure, especially in enclosed spaces, will cause permanent hearing damage.
Factors Affecting Overpressure
Overpressure is a byproduct of several factors including, but not limited to: barrel length, cartridge pressure, case capacity, powder type, and muzzle velocity. For instance, a 5.56 rifle discharged in a vehicle or a bedroom is going to intensify the blast in that confined area versus a 9mm pistol. The 5.56 rifle has around 58,000 PSI whereas the 9mm will be around 35,000 PSI.
The chamber pressure at discharge isn’t the only factor affecting overpressure. For instance, a 5.56 is around 58,000 PSI whereas a .50 BMG is around 55,000 PSI. If chamber pressure was the only factor, the .50 BMG should arguably have less overpressure than the 5.56. Common sense and experience dictate this isn’t the case. This is where case capacity (or powder volume) plays a factor. A larger case will have more powder. More powder means more volume and, as a result, more air displaced by the expanding powder. Gunpowder is an explosive when compressed and causes displacement of the surrounding air as it’s ignited. If there is more gunpowder, it will displace more air volume upon exiting the barrel. Furthermore, the burn rate of a particular powder has some effect on this. For instance, slower-burning powders, like shotguns and pistols, will not have as high of pressures as a rifle cartridge, which normally has faster powder burn rates.
Barrel length is a factor in overpressure. A shorter barrel will allow the expanding gases to escape closer to the shooter and while powder combustion is still occurring. For example, have you ever shot a carbine in a certain caliber and found it pleasant to shoot? How about if you shot that caliber in a handgun? The most common example of this is with .22s. I’ve shot rifles chambered in .22 Long Rifle without hearing protection and, while it can still cause hearing damage, didn’t find it unpleasant. Conversely, shooting that same cartridge out of my Smith and Wesson Model 18 or a Ruger 22/45 was profoundly different and caused an immediate ringing in my ears to my displeasure and discomfort. Longer barrels can, to some degree, help with reducing felt overpressure.
I mention these factors with the knowledge that the operating environment is a critical factor. For instance, I’ve discharged pistols, shotguns, and rifles from inside vehicles, rooms, under awnings, and in open air. Each of these environments has a different sensation generated by the discharge of the firearm and subtle changes such as opening a door, changing position relative to the firearm, and lowering the windows on the vehicle can substantially change the experienced overpressure for the shooter and those around them.
The Dangers of Overpressure
Overpressure has significant long-term consequences. Recently, there has been some discussion as to the possibility of “micro” traumatic brain injuries occurring due to the discharge of highly overpressure firearms in enclosed spaces. I’ve not delved into the available data or studies to discuss that issue further, but I will address the obvious issue of permanent hearing damage.
Hearing damage is a real concern and is exacerbated by the discharge of a firearm in confined spaces. When the human brain is stimulated by a threat or experiences a sympathetic nervous system response (e.g. adrenaline dump), the brain focuses its senses on the threat or stimulus presented.
For those of you who hunt, how many remember hearing the report of your .30-06 or similar high-powered rifle when focused on keeping that trophy buck in the crosshairs? This is your brain focusing on the stimulus (commonly visual) and ignoring auditory cues. This is why auditory exclusion is common in officer-involved shootings. With prior exposure to gunfire in a professional capacity, the gunshots sounded like muffled pops during the incident. While it didn’t have the immediate painful result I experienced when discharging a firearm on the range without hearing protection, the faint ringing in my ears in the nights thereafter indicated that I had experienced permanent hearing damage as a result of those incidents. The brain may mitigate the issue under stress but the damage still occurs nonetheless.
Mitigating Overpressure
For your personal defense plan, overpressure should be addressed. While keeping an AR-15 at the ready may be a high-capacity option with good terminal ballistics, what about the potentially life-altering effects of firing this inside your house? The porting on your handgun or rifle muzzle brake may reduce muzzle jump, but what issues could it present for hearing damage? If it isn’t your hearing, what about your loved ones? Have you ever fired these weapons in an enclosed space? If not, don’t let the first time be during a defensive scenario when the muzzle blast is something you’ve never experienced before. A full-powered rifle discharged in an enclosed space, especially a short-barreled rifle, is impressive in its report and blast.
There are a variety of ways to reduce overpressure for home defense firearms. I believe one of the best available options is using a pistol caliber carbine. The increased velocity of a rifle barrel increases terminal performance while reducing the overpressure experienced from a rifle cartridge. Conversely, the addition of a suppressor to your home defense firearm is strongly recommended and is an excellent mitigator of overpressure. Suppressors reduce the report, muzzle flash, and overpressure from the firearm. They also have the added benefit of reducing felt recoil in most instances. Of further consideration, subsonic cartridges like .300 Whisper or subsonic 9mm have less overpressure than their supersonic counterparts.
While finances may limit your ability to invest in accessories like a suppressor, mitigating overpressure is something that should be addressed in your personal defense plan. Overpressure can be mitigated through the firearm caliber, firearm type, and ammunition selected. Ultimately, as always, it’s upon you to find what works best for your needs and train accordingly with what you’ve selected. Find what works — or doesn’t — so you don’t have unintended consequences that could have been mitigated with some forethought and planning.
