Subsonic rounds have always been a subject which has been surrounded by mystery or misunderstanding. They are also somewhat complicated from a technical aspect which may not be widely appreciated. Here, Hexagon Ammunition’s Paul Bradley breaks down what Subsonic rounds are, why one might use them, how they are made and the pros and cons associated with them.
The term subsonic means “slower than the speed of sound”. That is where our first technical issue begins! The speed of sound is not a fixed parameter it is affected by temperature. We generally here that the speed of sound is 343m/s (1125fps). This is a standardized figure based upon a temperature of 20c. If we were to dial the temperature down to -20c the speed of sound falls to around 318m/s. If we go up in temperature, we see speed of sound increase to around 360m/s. Quite a deviation and certainly something to consider depending on time of year and geographical location. Some regions may have temperature differences of up to 40c between day and night.
So why make a bullet travel at subsonic velocity? Mainly to reduce the sound signature. Objects travelling at supersonic speeds (above the speed of sound) will create a loud noise. You may have experienced a jet making a sonic boom or even the crack of a whip. The noise you hear is the result of soundwaves being compressed into a single shockwave. The shockwave forms a cone with the speeding object at its tip. Anyone in the path (behind it) of the conical shaped shockwave will hear it as it passes them. Despite what many may think, firearms generally make three quite loud noises – The hot gases leaving the muzzle (often referred to as the muzzle blast), The sonic boom of bullets (if you are positioned to hear it) and the sound of impact (dependant on target medium). Using a sound suppressor greatly reduces noise from muzzle blast by slowing down the hot gasses. It does not reduce the supersonic boom of the speeding bullet at all. The only way to eradicate that is to slow the bullet down.
You may care about reducing noise for a variety of reasons. Maybe you are a tactical user (Mil/LE), maybe you are a hunter who wants to avoid scaring off nearby animals. It might even be that you just want a quieter shooting experience yourself. Using both a good sound suppressor and subsonic ammunition is the obvious solution. There is a secondary benefit to shooting subsonic which is recoil reduction. Recoil is force produced by mass moving at velocity in the opposite direction. Reducing velocity will reduce recoil considerably which makes for a very pleasant shooting experience.
We all know that you don’t get anything for nothing when it comes to ballistics. So there has to be some downsides to running subsonic loads. The major con is trajectory and range. The slower bullet will have a lot more drop at distance, the trajectory will look like a much bigger arc if you were to view it side by side with a supersonic on paper. This will be a lot less forgiving when it comes to range estimation and elevation adjustment. You really have to be on the money. Subsonic ammunition also tends to be a little less accurate. This is mainly due to velocity spread due to propellant volume. Most supersonic rounds have very little space left in the case when propellant is added. A subsonic round requires far less propellant which leaves you with a void. The propellant can move around in the case which can produce inconsistent burn rates and thus inconsistent velocities. Slower bullets are also more wind effected, this can be a pro or a con dependant on the context. If you are training to read wind it is a great benefit. You can replicate wind effects normally seen at much longer ranges and prepare yourself for that without the added cost of long-range hire. The con is if you are hunting or target shooting in high wind! You had better get pretty good with your windage adjustments!
There are also some dangers associated with Subsonic ammunition. The first of which is that it doesn’t “seem” dangerous when a beginner shoots it. The lack of noise and recoil can lead people to forget they are shooting a lethal projectile from a firearm. Instructors should make students aware of this. The most common danger are projectiles stuck in barrels. I have seen this happen several times, mostly with reloaders but not exclusively. There is a relatively fine line between having enough pressure to reach subsonic speed and having to little which leaves a bullet trapped in the bore. Firing again in this situation is often catastrophic for the firearm and sometimes to the operator. Reloaders should start at a charge weight suggested for average supersonic performance and work downward from there. I have seen 300BLK ammunition built specifically to give subsonic performance in short barrelled carbines become stuck in a long barrel bolt action rifle. It is something to be aware of. Lastly there is “secondary explosive effect”. This could occur when there is so much room in the case and so little propellant that it doesn’t burn properly at all. The smouldering propellant could then release highly explosive gases which will detonate at very high pressure. I have never witnessed this myself. The case capacity issue is a reason why many calibers remain difficult or unsuitable for subsonic work. We are unlikely to see subsonic .50cals gaining any popularity as the case would be to spacious internally to make a small propellant charge weight viable or safe (although you can with some modification).
Of course, as manufacturers we have to take all this into consideration. Your customers may be using many different barrel lengths. If you produce a round just under supersonic velocity at 20c for a short barrel rifle then it may be way over velocity in a longer variant. It may also get stuck in the bore as mentioned above. We therefore tend to be very selective over which calibers have subsonic options. The 300 Blackout is a case in point. It was originally designed as a subsonic caliber (because 5.56×45 is not suitable). It has a low case capacity and the dimensions can accommodate a long projectile which also takes up case capacity nicely. Of course, you pay in other ways for that as it makes a rather unimpressive supersonic option (not enough case capacity for decent velocity). Propellant choice is vital when it comes to a safe subsonic which performs well. Propellants with a high surface area which take up plenty of room are desirable, on the downside these will always be high burn rate variants. A manufacturer must try to play in the middle ground of all these variables to produce ammunition which gives acceptable performance for different customers.
We’d love to hear your thoughts on this topic, e-mail your views over to editorial@twsgroup.com
You can read the full February 2025 issue below:
