Muzzle Can or Over-Barrel Moderator?

What is the Difference?

Moderators are generally constructed using tubes of various types of materials, incorporating internal components to deal with the high pressures and the excessive sound pressure levels (SPL) experienced whilst operating firearms, which are harmful to the ear.

There are two main types of moderators; muzzle-mounted and over-barrel models. Both types have their advantages and disadvantages, however, both reduce the noise level of a gunshot.

Muzzle-mounted models (Muzzle Cans)

Muzzle cans are generally the more efficient of the two types as they do not have an internal tube like the over-barrel models, which reduce the capacity, thus keeping them shorter and lighter. Most commonly they are fastened to the end of the barrel by way of a machined thread.

Their only disadvantage is that they add considerably to the overall length of the rifle barrel, sometimes making it difficult to manoeuvre through dense bush; however the most common remedy is to shorten the barrel to accommodate the extra length.

Over-Barrel models (Telescopic)

Over-barrel models are generally longer than muzzle cans due to the increased length required to gain the similar internal volumes, which is lost due to the inner tube assembly. This type is attached the same way as the muzzle can; however it incorporates a sealed expansion chamber which extends back and over the exterior of the barrel. This chamber aids in keeping the exterior of the barrel free from burnt powder residue. They are also designed to cater for fluted barrels.

The over-barrel models have a lot shorter over-hang, and the rear of the moderator moves back over the barrel towards the fore end which gives a better balance than the muzzle can models. Over-barrel models are generally more expensive.

How Do Moderators Work?

Moderators are instruments designed for reducing the sound signature of a gunshot with the emphasis on reducing the risk of hearing loss.

As a shot is fired the precursor wave is the first to enter the moderator, followed by the projectile, then finally the blast wave, the main cause of hearing loss in shooters.

The recoil is generated by the combusting propellant, applying pressure to the rear of the projectile to gain the velocity required. As the projectile leaves the barrel it then passes into the blast baffle (Static Valve) which is one of the first components to come into operation.

The Blast Baffle serves three different purposes:

1: It is designed to help strip the high pressure gases away from the projectile (which can influence its flight) leading to accuracy issues.

2: The projectile partially seals the entrance to the blast baffle for a very short period of time (measured in milliseconds) allowing the blast wave/ gases to move rearward down inside the expansion chamber. This gives the projectile time to exit the moderator without the influence of the high pressure gas.

3: The blast baffle also acts as the muzzle brake which almost simultaneously occurs as the projectile enters into the blast baffle. It is the high pressure of the blast wave acting in conjunction with the specific weight of the projectile which gives the felt recoil. This pressure wave is then transformed to work in the opposite direction and apply forward force onto the face of the baffle which pushes the firearm away from the shooter thus reducing the felt recoil.

Moderator Baffle Stack

As the projectile moves into the forward section of the moderator it enters the baffle stack which has two functions:

1: The baffle stack helps to strip and deflect much of the precursor wave away from the path of the projectile.

2: As the projectile exits the moderator the blast wave (which is moving at approximately 2-3 times the speed of the projectile) is temporarily redirected to the filling of the rear expansion chamber. As the gas starts to flow into the forward section of the moderator the blast wave is met with the baffle stack which then helps the gas to further expand and cool which dramatically reduces the energy in the gas and so reduces the sound pressure level.