Muzzle Can or Over-Barrel Moderator?

What is the Difference?

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

Muzzle can models attach to the muzzle of the rifle barrel by a machined threaded section. These models are used on firearms where the amount of exposed barrel cannot accomodate an over-barrel model.
Muzzle cans are also priced lower than their over-barrel counterparts, this is due to less materials required in their manufacture.

Muzzle cans although cheaper are thought to be less effective than their counter part, however this is not the case. They have an immediate expansion chamber forward of the muzzle which allows for immediate expansion and cooling of the gunshot. The result being very good sound reduction, equal and in some cases better than the over-barrel models.

Over-barrel models attach to the muzzle of the rifle barrel by a machined threaded section. These models have a shorter muzzle over-hang resulting in the centre of gravity moving back towards the shooter allowing for a better balance.

The over-barrel models have an internal isolator tube, which encompasses the barrel and creates a sealed expansion chamber within the moderator. This prevents any gases from exhausting onto the exterior of the rifle barrel. The added internal components increase the cost of the product.

Unlike the muzzle can the over-barrel models have two points of contact on the barrel, the rear section of the moderator has a soft bushing which is machined to a precise fit to the barrel diameter at the point of contact.

This allows for controlled, uniformity of the barrel harmonics as the firearm is discharged, increasing the firearms potential for better accuracy.

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 pressure wave, the main cause of hearing loss in shooters.

Moderators are designed to hold back the energy from the gunshot, allowing the gases to expand and cool, reducing the energy.

This in turn reduces the Sound Pressure Level (SPL).

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.