Sound Moderators

By Peter Gethin

Sound moderators, sound suppressers, and silencers are just some of the terms used to describe the typically tubular device that attach to the muzzle of a firearm with the aim of reducing the weapons discharge signature. Moderators are available for centerfire and rimfire rifles and pistols as well as shotguns.

As most firearms launch supersonic projectiles it is worth noting that "silenced" weapons are not at all silent, the term moderator or suppresser is a more suitable title as this better describes the function of the device. Even with a silencer there will be the characteristic sonic boom created by the projectile itself as it accelerates through the sound barrier. This is unavoidable unless subsonic ammunition is used with its accompanying lower energy/velocity and resulting rainbow trajectory.

When a firearm is discharged it is not only the projectile that exits the barrel, but also all of the hot expanding gasses generated by the propellant. Upon exiting the muzzle these hot propellant gasses rapidly expand in the surrounding air resulting in a loud bang and a variable degree of muzzle blast. These expanding gasses, as well as the projectile itself, are also responsible for the recoil characteristics of the firearm. It is this expansion bang that the moderator is designed to minimise, in doing so it effectively reduces both muzzle blast and recoil.

The design of moderators varies greatly, with manufacturers producing ever more efficient units (so they say). But the underlying concept and layout has remained the same. The moderator is simply a hollow tube threaded at near end for attachment to the muzzle, with an expansion chamber in the middle and a series of baffle plates with just-larger-than-calibre concentric holes in their centres at the far end. To maximise the efficiency of the moderator the holes in the baffle plates need to be as small as possible whilst allowing the projectile to pass through unimpeded. As a result the projectile does not make contact with the moderator at any point during its journey.

The use of a moderator does not affect the power, velocity or trajectory of the projectile in any way. Even so, the added weight at the muzzle does alter the balance of the rifle and will therefore alter the point of impact, requiring the rifle to be re-zeroed after a moderator is fitted.

The efficiency of a moderator typically depends on the size of the expansion chamber and the number and arrangement of the baffles. There is also a calibre consideration; smaller calibre weapons such as rimfires require a smaller expansion chamber, as the large chambers of the centerfire moderators tend to resonate with these small rounds. Centerfires generally require larger chambers to soak up the greater quantity of gasses produced by the bigger propellant charge. Modern moderators are often designed specifically for calibre groups of similar power; this maximises the efficiency of the units by matching the chamber size and baffle arrangement to the quantity and pressure of gasses produced by each calibre group.

When a moderated firearm is discharged the projectile exits the barrel as usual, passes along the expansion chamber, and exits through the central holes in the baffle plates on its way to the target. At the same time a large proportion of the following propellant gasses are diverted and contained within the expansion chamber, where they are allowed to expand, cool and slow down. These gasses then pass through the central holes in the baffle plates where they expand and cool even more in each baffle cavity. Finally they exit the moderator with a lot less energy than when they exited the muzzle, significantly reducing noise, muzzle blast and recoil. This expansion chamber/baffle arrangement is the same system that is used in many automobile mufflers.

When looking at moderators there are two different physical layouts, firstly the conventional muzzle-mounted moderator and secondly the less conventional reflex or telescopic moderator. The muzzle-mounted moderator as its name suggests, is attached to the end of the muzzle; thus the expansion chamber and baffles are all housed off the end of the muzzle. This greatly increases the overall length of the weapon; as a result this incurs both balance and handling penalties. The reflex moderator sleeves the expansion chamber back over the barrel greatly reducing the overall length - having only the baffles in front of the muzzle, this improves balance and handling as well as offering a more secure two point attachment to the weapon.

The vast majority of production rifles are not pre-threaded at the muzzle to fit a moderator; this work has to be carried out as a customisation. Therefore it is necessary to determine the type of thread needed to fit the moderator of choice, which will have a standardised thread type. Then have the muzzle of the rifle properly threaded and possibly re-crowned by a competent gunsmith.

Once the barrel has been threaded and the moderator attached, it must be kept tight. Multiple shots will cause the moderator to work loose over time, which can result in the moderators bore falling out of alignment with the bore of the barrel with disastrous consequences.

Another important consideration on safety grounds is the rifling twist rate in comparison the projectile weight. If the twist rate is too slow to properly stabilise the projectile of a given weight, then it may begin to yaw the moment it exits the muzzle. This would be very hazardous, as the holes in the baffles are only just larger than calibre size.

The addition of a moderator to a rifle will add both length and weight, which are the two greatest drawbacks. This maybe an important consideration for field shooters who have to carry their rifles around all day and often require faster handling. This is not such a problem for the varmint or target shooter who typically shoots from a supported position with a long/heavy barrelled rifle.

The sound moderator offers a number of distinct advantages to the shooter: reduced sound, recoil, muzzle blast and muzzle flash. The reduction in sound means that fewer people are disturbed by shooting activities, which is particularly important when shooting near to populated areas or at night.

Also, the reduction in sound means there is less potential for hearing damage to the shooter. To the hunter in the field the sound reduction provided by a moderator means that no hearing protection is necessary and nearby quarry is not spooked by the muzzle report. It is easier to locate and shoot quarry that doesn't know its being shot at!

The reduction in recoil makes the weapon far more pleasurable to shoot, particularly with high volume shooting, and increases the potential for accuracy. This is equally as important to the hunter and the target shooter. The reduction in muzzle blast makes it less likely that the shooter will flinch, particularly with high powered rifles. Also less muzzle blast is good when doing chrono measurements, as I have killed three chronographs using high powered cartridges with high muzzle blasts (due to me positioning the chrono too close to the muzzle).

The reduction in muzzle flash is of particular use to the night hunter; the moderator will typically eliminate or at least attenuate the muzzle flash produced from virtually any cartridge. One final advantage is that a sound moderator will improve accuracy and consistency shot to shot as the projectile exits the muzzle into still air within the moderator's chamber. This region of still air means the projectile always begins its flight in controlled and repeatable conditions, without the interference of any side winds or sudden gusts.

The addition of a BR Tuote T8 (Eight Baffle) Reflex moderator to a Steyr Pro-Hunter MKII (SBS 96 Action) in .308 Winchester results in a 22dB sound reduction, which is equal to a massive 92% decrease in sound at the shooter ear. This is accompanied by an approximate 40-45% decrease in felt recoil, which is a real bonus for this light rifle. This is just an example, and specific for this calibre, but these results are typical of the results achieved with a good quality sound moderator.

There are laws in place in most countries regarding the use of sound moderators on firearms. Before purchasing or using a sound moderator make sure that your local laws permit their use, and if necessary apply for a permit. Also make sure the silencer is approved for the weapon on which it is to be used, as moderators are precision devices that are made to specific tolerances.

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Copyright 2004, 2013 by Peter Gethin. All rights reserved.