Muzzle brakes and recoil compensators are devices that are fitted to the muzzle of a firearm or cannon to redirect propellant gases with the effect of countering both recoil of the gun and unwanted rising of the barrel during rapid fire. Muzzle brakes are very useful for combat and timed competition shooting, and are commonly found on rifles firing very large cartridges (often big-game rifles), as well as some artillery and tank guns. They are also commonly used on pistols for practical pistol competitions, and are usually called compensators in this context.
Muzzle brakes are simple in concept. One of the simplest designs can be found on U.S. 90 mm tank guns. This consists of a small length of tubing mounted at right angles to the end of the barrel. Brakes most often utilize slots, vents, holes, baffles, and similar devices to redirect and control the burst of combustion gases that follows the departure of a projectile. Another method, called porting involves ports or holes in the barrel near the muzzle that vent gas prior to the departure of the bullet. A third method involves slowing the departure of combustion gases rather than redirection. Slowing of the gases is the method used on suppressors and linear compensators. In conventional designs, combustion gases depart the brake at an angle to the bore. This counteracts the rearward movement of the barrel due to recoil as well as the upward rise of the muzzle. The effect can be compared to reverse thrust systems on aircraft jet engines. The mass and velocity of the gases is significant enough to move the firearm in the opposite direction of recoil. On the AKM assault rifle, the brake is angled slightly to the right to counteract the sideways movement of the gun under recoil.
More advanced designs use baffles and expansion chambers to slow down the escaping gases; this is the basic principle behind a linear compensator. Ports are often added to the expansion chambers, producing the long, multi-chambered recoil compesators often seen on IPSC raceguns.
There are advantages and disadvantages to muzzle brakes. Recoil is a subjective concept. One shooter may perceive it as pain, another as movement of the sights, and another as rearward thrust. Recoil energy can be sharp if the impulse is fast or may be considered soft if the impulse is slower, even if the same total energy is transferred.
Besides reducing felt recoil one of the primary advantages of a muzzle brake is the reduction of muzzle rise. This allows a weapon's sights to be realigned more quickly. This is relevant particularly for fully automatic weapons. Muzzle rise is often entirely eliminated by an efficient design. Because the rifle moves rearward less, the shooter has little to compensate for. This is particularly true of rapid-fire, fully-automatic fire, and large-bore hunting rifles. They are also common on small-bore varmint rifles, where reducing the muzzle rise allows the shooter to see the bullet impact through a telescopic sight. A reduction in recoil also reduces the chance of undesired (painful) contacts between the shooters head and the ocular of a telescopic sight or other aiming components that have to be positioned near the shooters eye. Another advantage of a muzzle brake is a reduction of recoil fatigue during extended practice sessions, enabling the shooter to consecutively fire more rounds accurately. Further, flinch (involuntary pre trigger release anxiety behaviour resulting in inaccurate aiming and shooting) caused by excessive recoil may be significantly reduced or eliminated with certain shooters.
The advantages of brakes and compensators are not without cost, however. The most obvious of these to the shooter or gun crew is the increase in sound pressure level as well as the increase in muzzle blast for the shooter or gun crew. This occurs because the sound, flash, and pressure waves normally projected largely away from the shooter are now partially-redirected outwards to the side or even at backward angles towards the shooter or gun crew. While eye and ear protection should always be used when shooting, this is not even enough to avoid hearing damage with the muzzle blast directed back towards the shooter or gun crew.
"Recorded noise levels (on certified audiological instruments) at the muzzle of a magnum or high velocity rifle with a muzzle break [sic] normally exceed 160 decibels. Permanent ear damage occurs at 120 decibels. If you read the fine print on the finest set of ear plugs and ear muffs available, you will find the total noise reduction only between 22 and 31 decibels. This means that on a rifle with a muzzle brake, even if you are wearing hearing protection, you are suffering permanent damage.
Measurements indicate that on a rifle a muzzle brake adds 5 to 10 dB to the normal noise level, increasing total noise levels to 160 dB(A) +/- 3 dB. Painful discomfort occurs at approximately 120 to 125 dB(A), with some references claiming 133 dB(A) for the threshold of pain. Active ear muffs are available with electronic noise cancellation that can reduce direct path ear canal noise by approximately 17-33 dB, depending on the low, medium, or high frequency at which attenuation is measured. Passive ear plugs vary greatly in their measured attenuation, ranging from approximately 20 dB to 30 dB, depending on whether or not they are properly used. Using both ear muffs (whether passive or active) and ear plugs simultaneously is a practice that is often used for obtaining the maximum protection, but the efficacy of such combined protection relative to preventing permanent ear damage is not conclusive, with evidence indicating that a combined noise reduction ratio (NRR) of only 36 dB (C-weighted) is the maximum possible using ear muffs and ear plugs simultaneously, equating to only a 36 - 7 = 29 dB protection against a 160 dB(A) noise level. Relative to a noise level of 160 dB(A), this means that even using ear muffs and ear plugs simultaneously cannot protect a shooter against permanent ear damage when using a muzzle brake, through leaving a shooter exposed to noise levels of approximately 131 dB(A) that is 11 dB above the point where permanent ear damage occurs.
In the European Union (EU), employees are protected by law against impulsive or impact noise exposure over 137 dB(A), with an absolute limit of 140 dB(A). Individual member states often define even lower levels by national law; the United Kingdom, for instance, opted for 120 dB(A), which is where many references indicate that permanent ear damage occurs. The duration of exposure is likewise limited, for environmental noise above 90 dB(A) in the United States by OSHA rules, and for environmental noise above 80 dB(A) in the European Union, for exposure over an entire work day. The EU legislation demands the noise has to be reduced at the origin in the technically best possible way. Professional small arms and artillery users such as military and police personnel, etc., have to be issued with adequately working suppressors or hearing protection to reduce noise to levels as defined by law, since an employer has to anticipate legal action and compensations in cases where an employee that used the provided protective gear suffers work-induced health damage.
Brakes and compensators are additionally often quite bulky, adding length, diameter, and mass to the muzzle end of the firearm, where it will most influence its handling. Further, flinch caused by blast induced sinus cavity concussion discomfort experienced by the shooter can become a problem for anti-materiel rifle shooters due to the big cartridges these rifles fire.
A serious tactical disadvantage of muzzle brakes on both small arms and artillery is that, depending on their designs, they may cause escaping gases to throw up dust and debris clouds that impair visibility and reveal one's position. Troops often wet the ground in front of antitank guns in defensive emplacements to prevent this, and snipers are specially trained in techniques for suppressing or concealing the magnified effects of lateral muzzle blast when firing rifles with such brakes. Linear compensators and suppressors do not have the disadvantages of a redirected muzzle blast; they actually reduce the blast by venting high pressure gas forward at reduced velocity.
Muzzle brakes were ruled "legal" by the ATF in the United States a short time after the now defunct Federal assault weapons ban went into effect in 1994. These muzzle attachments were legal to attach to a threaded barrel, so long as they were welded in place on certain firearms (silver solder also sufficed). On the other hand, flash suppressors, and barrel shrouds were seen as 'military' features, and were on the list of features that, provided enough features were also present on the semi-automatic rifle, then the rifle was defined as illegal, if manufactured after the effective date of the ban. This meant, in practice, muzzle brakes had to be certified by the ATF to prevent end-users from accidentally violating the law and installing a device that could later be found to be defined legally as being more of a flash suppressor than a muzzle brake, even if marketed and sold as a muzzle brake, if incidental flash suppression was deemed to be 'significant' by the ATF. The Federal laws governing this sunsetted in 2004, and are no longer of active concern, except in those few jurisdictions in which certain provisions are still retained in specific state laws.