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Going Subsonic

Going Subsonic

When an object, be it an aircraft or a bullet, exceeds the speed of sound (1100fps in dry air at sea level) the rapid displacement of the air molecules causes a loud noise called a sonic boom. This is heard as a loud ‘crack’ in the case of gunfire, and it is why we wear hearing protection. If a bullet is travelling at less than 1100fps, there is still a significant sound, but this can very easily be controlled and reduced with the use of a sound moderator.
The use of moderators is common amongst stalkers, but some gun clubs now ask shooters to use them on the range, often because of complaints about noise from neighbours. The use of moderators goes hand in hand with using subsonic ammunition, and you can either buy subsonic factory ammo or you can reload your own. However, there are several factors to consider when it comes to the latter.

Reducing velocity
In theory, reducing the muzzle velocity below 1100fps is a case of simply decreasing the amount of powder, and then monitoring the muzzle velocity until it reaches the desired level, but there are several potential problems to consider.

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Too slow
When a round is fired, the powder charge rapidly accelerates the bullet through the barrel, forcing it to engage with the rifling and spin. To overcome the friction and resistance in the barrel, the chamber pressure must be high enough to push the bullet up and out, otherwise it will get stuck and block the barrel. This has the potential to be a real issue when developing a subsonic load.
If the bullet is travelling too slowly for the rifling to impart enough spin onto it, then it will not stabilise or shoot accurately. This is a particular problem with boat-tailed bullets at longer ranges because they rely on being stabilised correctly to fly straight.
Inside the moderator
When the bullet exits the muzzle of the gun and enters the moderator, it is effectively travelling through a free space and starting its journey. If the bullet has not been correctly stabilised in the rifling, then it can easily start to yaw, where the heavier back end of the bullet starts to rotate sideways away from the line of flight. If this happens, then the bullet can strike either the baffles inside the moderator or the exit, resulting in a written-off moddy. When you are working with low-velocity ammunition, you must always test it without a moderator fitted first, to ensure the bullets are stabilising, and then try it with the moderator on.

Developing loads
There are a lot of tested and proven subsonic loads available from reputable powder manufacturers and these are the best place to start. They will include information like the length of the barrel and the twist rate of the rifling, which are vital factors to consider when adapting a load to your own gun. If you decide to experiment with one of these loads, then you should be careful to reproduce the load and match the barrel configuration as closely as possible or make allowances for any differences. Just because a bullet stabilises and shoots accurately from a test barrel, does not mean it will do the same in your gun.
If you take a standard velocity load from published reloading data and drastically drop the powder charge in search of a subsonic recipe, then this is unlikely to work. Load density is a critical factor in making consistent ammunition and, as a general rule, as the load density drops, the velocity variation increases. At relatively short distances, which is what subsonic ammunition is really intended for, these variations are less significant, but at ranges over 100m, you will certainly see a loss of accuracy. To minimise this potential problem, it is important to use relatively bulky powders so that a reduced charge still occupies at least 60% of the case.
Accurate subsonic reloads depend on a high level of attention to detail and consistency. For example, in high-velocity rounds reaching 2900fps, a velocity variation of 50fps is not very significant, but that same level of variation on loads doing 1000fps will significantly affect bullet drop and grouping.
If you are developing a subsonic load, you should only change the charge weight in very small increments, around 0.3 to 0.5-grains, so that you do not drop the velocity too much in one go.

Bullet choice
If you intend to use a subsonic load on game, then it is very important to remember that most expanding bullets will only function correctly at higher velocities, and their ability to expand and transfer energy to the target will be greatly reduced at subsonic speeds. If such a bullet hits an animal at under 1100fps, it is most likely to under-penetrate, fail to expand, and cause a non-fatal injury, which would be a disaster. Slower bullets need to be heavy enough to dispatch the target effectively, without the need to expand.
Long-for-weight bullets tend to have a far greater bearing surface engaging with the bore, creating far more frictional drag. A subsonic powder charge might get the first couple of rounds off safely, but as the bore gets dirty, you may find that the next bullet is unable to overcome the increasing friction against the bore’s surface and therefore gets stuck.
Shorter, more basic, heavy-for-length and ‘stubby’ bullets will help to overcome the above issues, as will purpose-built bullets like Hornady’s SUB-X, which are made specifically to expand at subsonic velocities.

The important thing to remember when reloading subsonic ammunition is that it is not just a case of getting the muzzle velocity below that critical speed of 1100fps. The bullets must have enough energy to consistently overcome the frictional resistance in the bore and stabilise in the rifling. On target, the bullet must have sufficient retained velocity and energy to achieve the desired effect and, in the case of live game, function as a one-shot stopper.