The Firing Sequence
When the trigger of a gun is pulled, a firing pin moves forward and strikes the primer in the base of the cartridge. The mechanical shock of this impact causes the priming compound to detonate, and the shock wave generated by this then initiates the main propellant charge, causing it to deflagrate. As the combustion products are constrained, the pressure rises very quickly – in a typical full fore rifle cartridge a pressure of between 50,000 and 60,000 psi can be attained. The expanding propellant gases push against the bullet, overcoming the crimping forces holding the bullet into the cartridge case, and force the bullet forward to engage the rifling in the barrel. As the propellant continues to burn, the combustion gases continue to expand and drive the bullet down the barrel. At the muzzle of the barrel the bullet emerges and is briefly overtaken by the faster moving gases, which can upset to it a greater or lesser degree and thus influence its accuracy. The ‘unplugging’ of the barrel as the bullet exits releases the pressure within the barrel. It is this rapid expansion of hot, supersonic, erosive and corrosive gases that creates the muzzle blast, and it is this muzzle blast that a moderator attempts to moderate. The noise created by the bullet breaking the sound barrier cannot be moderated.
Noise Moderation.
Most existing moderators are based on old designs (Maxim, circa 1905) and have been updated only slowly. The image below shows a moderator comprising an expansion chamber (right hand side) and a series of baffles.
Moderators work by utilising three principles:
- Expansion of the compressed gases
- Deflection of the pressure wave
- Cooling of the gases
The moderator (this term is better than ‘silencer’ – with few exceptions the noise is not entirely silenced, merely supressed or moderated) is typically a hollow cylindrical piece of machined metal (steel, aluminium, or titanium) containing expansion chambers that attaches to the muzzle of a firearm. These “can”- type moderators may be detached by the user and attached to a different firearm of the same calibre.
Moderators reduce noise by allowing the rapidly expanding gases from the firing of the cartridge to be briefly diverted or trapped inside a series of hollow chambers. The trapped gas expands and cools, and its pressure and velocity decreases as it exits the moderator. The chambers are divided by either baffles or wipes. There are typically at least four and up to perhaps fifteen chambers in a moderator, depending on the intended use and design details. Often, a single, larger expansion chamber is located at the muzzle end of a can- type moderator, which allows the propellant gas to expand considerably and slow down before it encounters the baffles or wipes. This larger chamber may be “reflexed” toward the rear of the barrel to minimize the overall length of the combined firearm and moderator, especially with longer weapons such as rifles.
Some moderators – ‘wet’ moderators – additionally include a small quantity of fluid, generally water (best) or oil within them further to assist in cooling the propellant gases.
Another type is the “integral” moderator, which consists of expansion chambers surrounding the barrel. The barrel is pierced with openings or “ports” which bleed off gases into the chambers. This type of moderator is part of the firearm, and maintenance of the moderator requires that the firearm be at least partially disassembled. The most effectively ‘silenced’ firearms are usually those that have purpose built integral moderators such as the DeLisle carbine or the Welrod pistol.
The DeLisle carbine, shown above, was manufactured in small quantities in the Second World War for use by British commando raiding parties who needed a silenced weapon capable of eliminating sentries and guards. Based on a Lee Enfield rifle action, it used the .45 ACP cartridge which is inherently subsonic and had an asymmetric integral moderator around its barrel. It is generally regarded as being one fo the few truly silent weapons ever made.
Above is shown a Welrod silenced pistol in .32 ACP
Handguns are often shown, particularly on TV and in films, with a moderator attached, even revolvers. However, revolvers are very difficult to silence. The gap between the cylinder and the barrel allows a very considerable portion of the propellant gases to escape without entering the barrel as can be seen in the image below. This produces not only a flash but a very large amount of noise.
One revolver that can effectively be silenced is the M1895 Nagant, developed by the Belgian designer Emile Nagant and which has seen extensive service in Russia. This design has a unique mechanism where the cylinder is cammed forward to to mate securely against the barrel before the hammer falls to fire a round. The ammunition too contributes to the process, the bullet being fully embedded in an elongated cartridge case. As the cylinder moves forward and meets the barrel, this elongated cartridge provides very effective obturation, sealing the barrel and ensuring all the propellant gases are channeled to the muzzle. Adding a silencer to the muzzle moderate this revolver as effectively as any pistol.
The ability to moderate the Nagant was further enhanced by the BraMit devices developed by the brothers Mitny. The Mitnys did in fact make more than a few suppressed firearm designs, to the point where later partisans would often call ANY silencer a “BraMit device”. Their initial design is pictured below, basically a modified 7.62 Nagant that fired saboted .22 calibre projectles; the revolver would seal the barrel/cylinder gap at the rear, and the muzzle was closed off with a pivoting drum that would catch the sabot, but allow the .22 calibre projectile to travel through the device. After the shot, the front drum could be rotated to eject the sabot out the back, and it would then be ready to fire again.
An alternative approach
When the Russian military and secret services required a silenced pistol to replace the Nagant, their designers took a different approach. Rather than attempting to silence the propellant gases, they decided simply to capture them within the cartridge. The result was the MSP – “Malogbaritniy Spetsialniy Pistolet” – Small Size Special Pistol. The first version was effectively a 2 barreled derringer. The cartridge case has a piston behind the bullet, which pushes the bullet and seals the gases inside the case! The shot is noiseless and no flash is generated from the muzzle. The calibre is 7.62mm, muzzle velocity is 150 metres per second (490 ft/s) and the gun weighs about a pound.
To illustrate this talk, the Royal Armouries provided a wide variety of moderated firearms including several silenced Sten guns, a De Lisle carbine, an extremely rare prototype Welrod pistol, a silenced Chinese Type 54 sub-machine gun and several examples of Russian MSP pistols.