How Does A Gun Silencer Work? | Notes And Queries | .uk

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SPECULATIVE SCIENCEHow does a gun silencer work?

  • THE CRACK of a shot is made by the bullet breaking the sound barrier, and the rapid escape of the propelling gasses from the muzzle of the gun. A silencer is essentially a tube containing baffles and a barrel sized tube with holes. The gases that propel the bullet are diverted through these holes, and into the baffled area. The baffles slow down the gasses to the point where there exit does not make as much of a sound. By using sub-sonic ammunition, one can reduce the report due to the bullet breaking the sound barrier, and increase the effectiveness of the silencer.

    James Norminton, Canterbury, Kent ([email protected])

  • SOUND is essentially the way that we perceive rapid fluctuations in gas (air) pressure. When gas of high energy (pressure, velocity or temperature) comes into sudden contact with much lower energy gas (typically the ambient atmosphere), sound is generated. Its loudness depends upon the magnitude/speed of the transition. A gun silencer or for that matter, a car silencer works by dissipating the energy contained by the explosive/combustion gases, typically by a series of "baffles", prior to them being exhausted to atmosphere. Modern jet engines achieve their quieter performance by sheathing the main jet efflux in a "tube" of air of intermediate energy, thus reducing the magnitude of the single transition into steps. Incidentally, it is very difficult to properly silence automatic weapons because the force of the explosive gases is used to load the next bullet. The gases thus escape from the breech area rather than the barrel and may not be dissipated gradually.

    (Dr) Ian Tuersley, Warwick Manufacturing Group, University of Warwick ([email protected])

  • GUN SILENCERS, as Dr. Tuersley has generally stated, work by gradually reducing the pressure and velocity of the gas behind the bullet so that, by the time it hits the atmosphere, the distribution of vibration it conveys into it is extended and flattened. This is what produces, in an effective silencer, a soft low thump on discharge rather than the usual sharp, high crack. Typically, the bullet passes through a series of cylindrical chambers separated by baffles (in some successful examples made of oiled leather!) after leaving the barrel proper. The gas expands and circulates into each chamber at progressively decreasing pressure, venting finally after the bullet over a much longer period. However, the volume of gas a gunshot produces is usually very large if it is to produce the required power. The peak pressure in a rifle chamber may reach a couple of thousand atmospheres; that in a handgun perhaps half this figure because, although their bullet velocity is only of the order of one-third to half that of a medium-calibre rifle, they typically only have a fifth to a quarter of the barrel length to accelerate in. Therefore, to vent to atmosphere at or near comparable pressure, a silencer would need to have a volume of one or two thousand times that of the weapon's chamber. This would normally evaluate to several litres; and the usual type of silencer seen in thrillers is therefore obviously far too small. So the short answer to the question: "How do gun silencers work?" is "Usually not very well!" Pistol silencers of a few centimetres diameter and a few tens of centimeters in length are both very limited in effect and prone to being blown off the end of the gun unless excellently well- fitted. They usually affect accuracy severely and, especially in the case of small self-loaders, make the gun impossible to aim by obstructing the sight picture. There is some further loss of effect in self-loaders by venting from the breech whilst it is cycling, but the cylinder/barrel gap in revolvers is at least as much of a problem. For standard handguns of 7-8mm. calibre and above, silencing is ineffective and impractical; more so as calibre increases. Silencing a supersonic round such as 9mm. or .357" Magnum is further complicated by the sonic crack generated by the bullet's passage through the air; with this type of weapon silencing can at best only camouflage the direction and distance of the firer. Nevertheless, silencers of this size and type are very effective on smallbore rimfire rabbit rifles, and have also been used successfully on .22" pistols, in both cases owing to the small volume of gas evolved fron the propellant. They are usually known more accurately as "moderators" in these applications To silence larger weapons properly often requires that the silencer is embodied in the construction of the weapon. One successful example was the De Lisle carbine, first built as a clandestine weapon during WW2. This was a Lee-Enfield rifle rebarrelled from the powerful .303" rifle round to the much smaller-volume and lower-powered subsonic .45" automatic pistol cartridge. The silencer fully enclosed the barrel and projected beyond it. The barrel was vented into the surrounding silencer over about half its length by rows of holes progressively increasing in size toward the muzzle. The gun retained the original bolt action so that the breech remained shut until manually opened. Having used a more recent copy of this, it appears that the claim made at the time, that the fall of the firing-pin was the loudest noise of discharge, was an exaggeration. The thing sounded a little louder than a fairground airgun, and its velocity was 15% lower compared to the same round fired from a standard pistol. This would have limited its effective range to 30-50 metres or so.

    Mick Burmeister, Alvechurch, Birmingham ([email protected])

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