Why you DON’T want to break the sound barrier
by B.B. Pelletier
I learned a lot since this report was published, back in 2011. Freom October to December of that year I did a test with a Whiscombe rifle that showed that pellets can break the sound barrier and still be accurate. What you are about to read is what the airgun community thought was true, back in August of 2011. I have inserted my remarks that are updates as of December, 2021, and they are bolded for you. The text that’s not bolded is the original report.
This report has been done in bits and pieces many times over the years, but I’m putting it together today because of a surge of new airgunners coming online. Many of them are older firearm shooters, but many others are younger shooters with no real background in the shooting sports. We’re seeing an upturn of fundamental questions in our social networks and through customer service representatives that tell us that this topic needs to be emphasized once again.
Speed of Sound & FPS
The sound barrier is a lot more familiar to people of my generation, because it was being talked about and always in the news when I was a youngster in the 1950s. Young folks don’t think much about it these days because supersonic flight is a foregone conclusion; but back in the 1940s, it hadn’t yet been achieved by a manned aircraft in level flight.
A couple pilots inadvertently broke the barrier in dives from high altitude during World War II when they were testing certain fighter aircraft, and one of them was Cass S. Hough, the grandson of the founder of Daisy and later a president of the firm himself. At the time, he was trying to solve a control surface problem with the twin-engined P38 Lightning fighter, so he took one to over 40,000 feet, nosed it over into a steep dive and might have become the first man to ever break the barrier in an airplane. I say “might” because almost every air force of that period has a similar story. There’s a plaque in England that commemorates that flight in 1943, but I’m sure there must be other plaques in other countries, as well.
Before I hear from all the engineers (except the aeronautical engineers) that a prop-driven plane cannot go supersonic because the propeller has to break the sound barrier long before the aircraft does, it is possible — when gravity assists the aircraft — for a prop-driven plane to go supersonic. It’s not a good thing. As Hough discovered, the subsonic control surfaces no longer work right at supersonic speeds, but it can be done. As a result of Hough’s flight, the P38 received a special “dive flap” control to help free the controls when the speed got too high.
The problem with the sound barrier is what happens as you approach it and then pass through. In short, a pressure wave of air builds in front of whatever is moving that fast. Normally, this pressure would then flow around the surfaces of the aircraft and be left behind — but at transonic speed, the air compresses and develops eddies and currents that play havoc with the control surfaces of the aircraft. The surfaces that work well up to a certain subsonic speed start to act odd when they reach the transonic speed, which is about Mach 0.9, or nine-tenths of the speed of sound in the conditions of the moment.
One bad effect of reaching the sound barrier is a buffeting that causes the entire aircraft (or pellet) to vibrate. Aeronautical engineers had to learn to design aircraft for supersonic flight while maintaining the ability to fly at subsonic speeds as well.
Now, let’s talk about pellets
Pellets don’t have adjustable control surfaces. They are what they are, so they like to fly at certain speeds, and in all cases with standard diabolo pellets (wasp-waisted with a hollow tail) that speed is subsonic. In fact, even the transonic region is bad since it’s the place where the buffeting starts.
This is why we do not want to shoot pellets at 1,000 f.p.s. Because 1,000 f.p.s. is always in the transonic region.
How many fps is the speed of sound?
The answer is: it depends. Just like how long is a piece of string? Things that change the speed of sound are the elevation above sea level, the ambient temperature and humidity. Elevation is subtle, because it also influences the air temperature. Temperature of the air is the most influential factor that affects the speed of sound, and I’ve learned that where I live the barrier can exist anywhere from about 1,060 f.p.s. and above. The usual speed of sound is given as about 1,125 f.p.s. when all conditions are “normal.”
You know the pellet has exceeded the sound barrier when you hear a sustained crack with the shot that cannot be attributed to the muzzle blast. Silenced firearms dramatically show off this sustained crack because the bullet is quiet at the muzzle and then returns an indistinct sound like distant thunder as it goes downrange.
But, it isn’t the sound that airgunners should be concerned with. It’s the accuracy, or rather the lack of it that is caused by the buffeting mentioned earlier.
[Editor’s note: This report caused me to do an 11-part accuracy test that I titled the Pellet velocity versus accuracy test. Part 11 was in December of 2011 and has links to all the previous reports. Now let’s return to the old report and see what was said.]
Apparently some people have asked whether bullets break the sound barrier. That is a question that’s too vague to answer. Are we talking about the bullets fired from a firearm? To give the question more definition I’ll ask whether the bullets from centerfire rifles break the sound barrier. The answer to that question is they do almost every time. Just for example, the very well-known 55-grain bullet from a 5.56mm US Army rifle goes over 3,000 f.p.s., which always breaks the sound barrier.
How pellets are stabilized
Pellets are stabilized both by spin and drag. Since they are hollow, they are light for their length, so the spin can be slower than for solid conical bullets. That’s why solid pellets are usually a failure. [Editor’s note: Solid pellets are now quite accurate, as the rifles that fire them are launching tghem at speeds that make their spin stabuilize the pellet (bullet).]
But pellets are also stabilized by high drag, just like darts. The wide hollow skirt creates a low-pressure area behind the pellet that drags on them as they fly forward. It keeps the point oriented forward and stabilizes the projectile in flight.
At subsonic velocities, [which means velocities below the sound barrier] pellets are usually stabilized pretty well; when they get up into the transonic region, they’ll flutter in flight, just like those older airplanes did. And, those flutters translate into larger groups. Knowledgeable airgunners like to keep their velocities under 900 f.p.s. for safety’s sake. [Editor’s note: This is still true in 2021. Airgunners feel that around 900 f.p.s. is as fast as they like to shoot when accuracy is on the line.]
One additional reason to stay below the transonic region
I was chatting with (my late friend) Mac about this; and we’ve both observed that in spring guns, the faster they shoot the twitchier they are as far as hold sensitivity. That has nothing to do with the sound barrier — it’s just a fact of life for spring guns. Throw in the breakbarrel design that’s also very hold sensitive and you have a real recipe for disaster. Yet when you look at all the magnum airguns that are being sold on the basis of velocity, the majority of them are breakbarrels.
So, we have a bad situation in which the most inexperienced shooters are drawn to the very airguns that are the most difficult to shoot on the basis of two things — the advertised velocity and the low cost! It’s like a church that decides to hold its meetings in the piano bar of a casino.
I’m on what, I guess, is a lifelong crusade to spread the word about airgunning so people don’t come in the wrong doors and find things amiss. I want to give each new shooter the same chance I had to discover the shooting sports on the very best terms. If they could just see a fraction of what I see, I know that many of them would be intrigued enough to stay and grow our hobby.
Airgunning can be fun and very satisfying if you do it the right way. The right way is to shoot enjoyable guns that hit their targets more often than not. Hyper-velocity airguns are the antithesis of that. They are the .338 Lapua Magnums whose owners have each fired one box of ammo before giving up on the beast.