Today’s topic of discussion is going to be the differences between supersonic, transonic, and subsonic speeds when discussing how projectiles move through the air. Why the photo of the 338LM for the featured shot? If you aren’t shooting out into Extreme Long Range (ELR) distances you’re likely only dealing with the supersonic portion of a bullet’s flight. You will hear people say things like “how far is the bullet supersonic?” but you may not hear the reason behind the question. What happens in the Transonic flight portion of flight? Does subsonic matter? We’re going to discuss what each speed is and, briefly, how they relate to shooting at extended ranges! So let’s get started!
You tend to hear the most talk about the supersonic portion of a bullet’s flight. There are good reasons for this. First off, this is the most predictable part of the bullet’s flight. Remember hearing stories about 308 Winchester not being able to shoot out to 1000 Yards? For a long time the belief was that a 168gr Sierra Matchking bullet just wouldn’t make it that far. The lore and superstition behind that belief have some basis in reality. For the vast majority of folks shooting at sea level and lower altitude the 168gr Matchking begins to transition into transonic flight around the 800yd mark. For years everybody just figured that as a rule of thumb, 800yds was about as far as you could reliably shoot a 308 Winchester. It was anecdotal in nature. There were accuracy issues at around the 800yd mark with the 168gr Matchking at lower elevations.
Then the 175gr Sierra Matchking came around. With a higher ballistic coefficient it was able to remain in supersonic flight to 1000yds reliably even at lower elevations. Suddenly the 308 Winchester is a viable 1000yd cartridge. All that really changed was the projectile. The reason behind it has to do with the effects of transonic flight on a bullet! The 175gr Matchking has a higher ballistic coefficient and is able to maintain velocity and energy better than it’s predecessor. As a result the supersonic portion of the bullet’s flight was extended and the effective accuracy limits along with it.
This is the portion of the bullet’s flight where the speed has dropped to around Mach 1.1 and continues to around Mach 0.9. In simple terms, that guys like me can make sense of, it just means the bullet is transitioning from supersonic to subsonic flight. The part to remember is when you fire a rifle and hear the crack what you just heard was the bullet breaking the sound barrier. It’s jumped to a point where it’s traveling faster than the speed of sound. That’s roughly 1100fps and varies according to environmental conditions.
While the bullet is supersonic it is moving faster than the sound it produced when it was fired. However, as it slows down over time and distance the sound will eventually catch up. When that happens the sound wave, shock wave, whatever you want to call it…will catch up and pass the bullet. When that happens it tends to destabilize the bullet’s flight. The bullet no longer flies predictably and accuracy typically suffers greatly. Hence the whole, “308 is only good to 800yds,” saying you’ve probably heard. That’s based on the bullet that was prevalent at the time entering transonic flight at around the 800yd mark.
This is also why you hear questions about the supersonic range of a cartridge. If I want to shoot to 1200 yards in a Precision Rifle Series style competition, I need to choose a caliber and bullet that will get the job done. Higher ballistic coefficient bullets at higher velocities will make it happen. 1200 yards isn’t that hard to reach with modern cartridges. Handloading also helps since you can tailor the load to the rifle and what it’s able to handle. Factory ammunition is more tame because it has to be manufactured to standards that make it safe in EVERY rifle. A handload can be customized to gain maximum performance out of YOUR rifle.
Subsonic flight is a lot like supersonic flight. The bullet moves through the air unimpeded. It’s traveling behind the sound produced by the rifle firing. It’s trajectory is pretty accurate and predictable. The difference is in muzzle velocity. At subsonic speed you have a steeper trajectory to attain the same distance. While velocity isn’t the end all for ballistic coefficients it does play a role. So the slower the bullet goes the more it’s ballistic coefficient can suffer. You see a lot of rifles shooting subsonic rounds these days in conjunction with suppressors in order to produce a really quiet rifle. The bullet is moving slower and they tend to be heavier projectiles in order to help keep the muzzle velocity below the point where it would become supersonic. The other advantage of the heavier bullet weight is a higher ballistic coefficient. While it helps with wind drift you still have a steeper trajectory than you would with a supersonic round.
Some bullets have been designed to make the transition between supersonic and subsonic better and with greater accuracy so the transonic portion of flight doesn’t result in a complete lack of accuracy at extended ranges. This has a lot to do with the bullet’s design and ballistics and the real meaty science behind bullet flight. It’s a bit above my level! I know Berger designed the 175gr OTM bullet specifically to handle the transonic portion of flight better than competing products.
The thing to remember is that for the vast majority of us, who only shoot to distances where the bullet is still traveling at supersonic speeds, we don’t have to worry about accuracy degradation during transonic flight. If all you deal with are ranges where your cartridge is still moving at supersonic speed than all the talk of transonic and subsonic flight are easily ignored. However, if you’re shooting ELR with a big bore rifle this comes into play in a bigger way. Higher ballistic coefficients on heavier bullets are desirable in conjunction with higher muzzle velocities to try and keep the bullet in supersonic flight longer. If you want to make the fabled “Mile Shot” you will likely have to contend with transonic flight. If you custom load your own ammunition and make smart choices with powders and projectiles, you may be able to keep the bullet in the supersonic portion of it’s flight to the One Mile boundary.
If you’re shooting to that kind of distance then selecting a bullet that handles transonic flight well without losing a lot of accuracy becomes really important. Lean on the data that’s out there from other folks that have already done it. Look for bullets that have been designed with shooting through transonic flight in mind. They’re hosting King of Two Mile shooting events now where people are shooting well into the transonic and subsonic regions of a bullet’s flight. We live in a time where we’re shooting farther than ever before and we’re hitting the target! If you have anything to add, drop it in the comments below!