Why don’t they design pellets to go supersonic?
by B.B. Pelletier
Whenever I write about the fundamentals of shooting, it usually starts a good discussion. The CB cap vs pellet rifle article spawned an article about why we like to keep airgun velocities under the transonic/supersonic level for the best accuracy, and THAT, in turn, evoked this thoughtful question on the Pyramyd Air facebook page last week:
“This may be a dumb question — but, since the issues revolves around the ‘badminton birdy’ design of our current air rifle pellets. Has there been any attempts to change the design to provide stable flight, and maintain more energy, at faster speeds? Just curious….”
That is not a dumb question at all! In fact, it’s such a good and thoughtful question that I thought it deserved a special report because we’re seeing a rise in the number of firearm shooters who are reading this blog. Just like airgunners, those who shoot firearms come with different levels of experience; and some of them are not attuned to the fundamentals of accuracy. They buy commercial or military surplus (milsurp) ammo and just shoot it without appreciating how much better they might do with a little tweaking.
The same can be said of airgunners, many of whom have bought into the high-velocity craze without realizing (or perhaps caring) all they are giving away. Today, I want to look at the projectile we shoot — the common pellet — with the hope that, by understanding its design and limitations, we can extract the best our airguns have to offer.
The diabolo pellet
Diabolo (pronounced dēˈabəˌlō). According to the dictionary, the origin of the word comes from a toy top that was popular in parts of Europe. It was also sometimes used in juggling performances. The word came from 20th century Italian from the ecclesiastical Latin diabolus, which means devil; the game was formerly called devil on two sticks.
The diabolo pellet is characterized by a pinched or wasp waist and a flared hollow tail or skirt. Though there are many different variations on this central theme, they all have these characteristics.
The diabolo pellet can have different nose shapes, but all of them have a pinched waist and a flared hollow tail. The center of mass is biased forward by the hollow tail.
The design of the pellet biases the center of mass forward of the center point, like a throwing dart. The flared skirt and to a lesser extent the pinched waist create high drag that keeps the pellet oriented forward in flight.
I wish I could say exactly when the diabolo pellet was first introduced, but I’ve been unable to find a source that gives a definitive date. Nor is there a George Diabolo after whom the pellet is named. What I can say at this time is that it didn’t exist in the 1880s but was already in existence when the first modern air rifle — the Lincoln Jefferies underlever made by BSA — was offered in 1905. That’s as close as I’ve been able to pin down the date of introduction. I would welcome any information that contradicts my dating or offers greater insight.
When the diabolo pellet was first sold, most airguns were smoothbores whose designs were already many decades old. Buglespanners, the underlever guns that cock via the triggerguard, were being made in calibers as small as .22 as early as the 1850s, though that caliber is rare. By the mid-1870s, a great many companies were selling smallbore airguns in many calibers.
Perhaps the most well-known and prolific of these, at least in the United States, is the Quackenbush company, whose proprietary .21-caliber long guns and pistols sold for a tenth the price of handmade gallery airguns from just a decade before. Quackenbush guns and the others like the Gem, Haviland and Gunn, and others all used darts and something called cat slugs (sorry, Edith) that were nothing more than cylindrical lead slugs of bore diameter. They were very short, so they either avoided the tendency to tumble or it didn’t matter that much. Another variation of the cat slug was the felted slug, which was a cat slug with a short wad of felt clued to the base to provide drag.
Once the diabolo pellet came on the scene, it quickly rose to the top of the sales heap, surpassing all other projectiles. It did so because its high-drag design stabilized the flight of the pellet without requiring a rifling-induced spin. However, spinning the pellets did much to improve their accuracy, and the new BSA spring guns could not have hit the market at a better time.
Where the other types of projectiles were inaccurate at distances beyond 30 feet (excepting some handmade darts that were extremely accurate and had been in existence for over a century, but required specialized and expensive dart guns), the new diabolos pushed out the distance to 60 feet, where they gave one-inch, five-shot groups. In that day, being able to group like that was like saying a modern PCP can group an inch at 200 yards. It was an unthinkable distance that revitalized airgunning like nothing before.
Diabolos and the accuracy barrier
Certainly, up to this point in time (1905), there had never been any thought given to airgun projectiles going faster than about 500 f.p.s.; and only that fast in very few guns in the smallest caliber (No. 1 bore, which is also called .177). Velocity was not important, as the airgun was seen as an extension of the gallery target gun — though one that was much less expensive and more available to the common man. Accuracy was the sole purpose for the diabolo until the mid-1920s, when the Crosman Corporation started selling a hunting-themed pneumatic (Power Without Powder).
Power/velocity in airguns crept up very slowly throughout the 1920s and ’30s, and accuracy did the same. What held back accuracy was not the barrels of the guns, some of which were very fine, but the quality of the pellets. Airguns had run into the “accuracy barrier” because the manufacturing processes hadn’t reached the levels they would several decades later. It wasn’t until after World War II that European pellet makers finally started making really accurate diabolo pellets.
Sheridan shows us the way
In fact, there’s an anecdote in all of this; because in 1947, the Sheridan company decided to not use a true diabolo design and instead created a proprietary cylindrical pellet that had no pinched waist but did still have an open tail. The tail was not flared; instead, it had a tiny stepped ring of lead that was slightly larger than the diameter of the rest of the pellet and that was what was engraved by the rifling when the pellet was loaded.
The vintage Sheridan cylindrical pellet was not a true diabolo, but it had high drag just the same.
The reason given for this departure was that there was no accurate .22 pellet available. That may have been the truth, because the first prototype Sheridan rifles were created in .22 caliber; though, when brought to market, they came in a proprietary .20 caliber that has been the same ever since.
The first Sheridan pellet was a throwback to the schuetzen rifle days when all lead bullets were made with bases that were a couple thousandths larger than the rest of the bullet. These bases sealed the bore against the hot gasses at firing, and they also made it possible for the shooters to load the bullets separately into the rifled bore ahead of the cartridge case. This prevented the bullet from tipping as it entered the bore, because it was already seated there by hand.
The one or two lead rings at the base of the bullet were relatively easy to engrave with the rifling, as opposed to trying to engrave the entire bullet. That was the mistake that British and German pellet makers made when they tried to make the solid pellets (which I’ll discuss in a moment).
The sound barrier is breached!
Until the 1980s, peak pellet velocities remained below about 870 f.p.s. In the early ’80s, several rifles finally achieved 1,000 f.p.s. Soon after that, British airgun designer Ivan Hancock broke the sound barrier with his Mach I breakbarrel springer that got over 1,150 f.p.s. in .177 caliber. After that, things changed very fast.
Suddenly, accuracy was out the window, as shooters discovered that the diabolo shape is not well-suited to flight in the transonic or supersonic region. The fact that the pellet remains at this high velocity for only a few yards makes no difference. The damage was done. The extreme buffeting caused when the pellet reaches and passes transonic speed, then slows back down and goes through it again is more than enough to destabilize it and cause groups to open.
Sales go crazy!
However, the other side of the coin is that high velocity sells guns. A company that advertises their gun shoots 1,000 f.p.s. and higher attracts lots of attention and, yes, sales. In fact, so much attention has been given to 1,000 f.p.s. that it is now seen as the marketing kiss of death to advertise anything less. Some companies have gone to great lengths to tout ever-higher velocities without a thought being given to accuracy. Special lightweight, lead-free pellets are now selling well partly because of the velocity boost they give to the guns that shoot them.
Which brings us back to the initial question
If the diabolo design doesn’t work at high velocity, and we know unequivocally that it doesn’t, then why doesn’t someone design a pellet that can exceed the sound barrier? Well, to a very limited extent and with disastrous results, it has been done. The so-called “solid pellet” was the first attempt to do this. This projectile is really a bullet — not a pellet, and as such is brings all its bullet weaknesses with it. The first is that nobody can load a lead bullet into the bore of a rifled gun unless he’s Superman. Those who shoot muzzleloaders know that it takes a device called a short starter and often a separate mallet to force the bullet into the rifling of a bore.
These .22-caliber Eley solid pellets weigh 30 grains and require the shooter to engrave the rifling at loading. They failed because they’re too difficult to load and because they’re inaccurate in most airguns. Other designs were similar and have had the same problems.
So, no solid pellet currently on the market can be loaded into an airgun easily enough to use. If it could, the second problem crops up. The twist rate of the rifling is too slow to stabilize a solid pellet. That twist rate, which is very often one turn in 16 inches of travel, was taken from the .22 long rifle cartridge when the first modern air rifle was made. It hasn’t changed since then. It works with diabolos, but not with solid pellets because they’re too heavy for the lower velocity at which most airguns can propel them. They have no additional means of stabilization and need to be driven faster to stabilize. Being both very heavy and also having a lot of friction with the bore, they go much slower in any given airgun.
Okay, make the airguns more powerful
About seven years ago, I could see where all of this was heading, so I tested these pellets extensively in an AirForce Condor — the only air rifle I can afford that can get them up to 1,000 f.p.s. You know what? They still aren’t accurate. They’re stabilized at that speed, but they still shoot in 5-inch groups at 50 yards, while diabolos going less than 950 f.p.s. will group in three-quarters of an inch from the same gun.
Okay, then why don’t “they” make a more powerful air rifle that can shoot these things really fast?
Stop right there!
Don’t you see where this is heading? When an AirForce Condor shoots a 30-grain solid “pellet” at 1,000 f.p.s., it isn’t an air rifle anymore. It has become a firearm in all ways except how it’s powered. The Condor can shoot a 30-grain diabolo that leaves the muzzle at 1,000 f.p.s. and probably kill a woodchuck at 75 yards with ease, yet it still won’t travel downrange any farther than about 500-600 yards max. The high drag of the diabolo design slows the pellet after a very short time, but a solid pellet leaving the muzzle of the same gun at the same velocity will go a mile and a half. It has nothing to slow it down. We’ve then turned the Condor into a .22 short.
There’s an airgun maker in the Netherlands that makes custom .25-caliber rifles that can shoot 60-grain jacketed boattail spitzer bullets at over 1,200 f.p.s. That’s very admirable for an airgun, but that rifle, my friends, is a .25-20 Winchester in all ways but the name. Maybe not the modern loading of the cartridge, but it’s certainly close to the original loading. So, while it can actually be done, I’m saying that it shouldn’t be. Turning an air rifle into a firearm is just asking for more legislation that we don’t need.
Now, before some of you go off on big-bore airguns, they’re just as relatively safe as smallbore airguns. They shoot about as far as shotguns shooting rifled slugs, and most states that worry about distance limits for sporting guns allow the shotgun with slugs.
It’s not the power of the gun at the muzzle, but how far downrange it throws the projectile that makes it more or less safe. And, with diabolo pellets, airgunners have achieved something truly remarkable — a safer bullet.
I hope this report sheds some light on today’s state of airgun technology.