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When pellets go supersonic: what happens when the diabolo pellet breaks the sound barrier?

It's difficult to buy an air rifle these days that doesn't offer at least 1000 fps--especially in .177 cal. But that puts the pellet out the spout at transonic speed, just short of the sound barrier, which is exactly where the air turbulence is the worst for accuracy.

Centerfire shooters who shoot long distances calculate the distance at which their bullets stop going supersonic and slow back down through the buffeting transonic region, because that is where their bullets lose accuracy Rimfire target shooters are all too aware of how damaging to accuracy supersonic flight can be.

What about the diabolo?

But what happens to a projectile that's specifically designed not to go supersonic, when it's boosted past the comfort zone? The diabolo pellet is designed to be a high-drag projectile that gains much of its stability in flight from the drag produced by the flared open skirt. It works much like an arrow in flight.

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 design of this type of 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 creates high drag that keeps the pellet oriented forward in flight.

A brief history

We know from magazine ads that diabolo pellets existed as early as 1902, but they don't seem to date much earlier. 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 best known and most prolific of these, at least in the United States, is the Quackenbush company, whose proprietary .21 cal. 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 used darts and something called cat slugs 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 glued to the base to provide drag.

Once the diabolo pellet came on the scene, sales rose quickly until it surpassed all other projectiles. The accuracy was clearly superior to the others. It was 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 1-inch five-shot groups. In that day, being able to group like that was like saying a modern PCP air rifle can group inside an inch at 200 yards. It was an unthinkable distance that revitalized airgunning like nothing before.

Certainly, up to this point in time (1905), there had never been any thought given to airgun projectiles fired from spring-powered guns going faster than about 500 fps, and only that fast in very few guns of 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's existence until the mid-1920s, when the Crosman Corporation started selling a hunting-themed pneumatic.

Power and velocity in airguns crept up very slowly throughout the 1920s and '30s, and accuracy did the same. What held back accuracy wasn't the barrels of the guns, some of which were very fine, but the quality of the pellets. Airguns had run into an "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 the way

In 1947, Sheridan decided to not use a true diabolo design and instead created a proprietary cylindrical pellet that had no pinched waist but still had an open tail for high drag. 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.

Sheridan thought there was no accurate .22 pellet available for their rifle. Had there been, we might never have seen the .20 cal. air rifle they pioneered. The first prototype Sheridan rifles were created in .22 cal. for function testing; though, when they were finally brought to market, they came in the now-familiar proprietary .20 cal. that has been their DNA ever since.

The sound barrier is breached

Until the 1980s, peak pellet velocities remained below about 870 fps, which is safely below the transonic region. In the early '80s, though, several rifles finally achieved 1000 fps. Soon after that, British airgun designer Ivan Hancock broke the sound barrier with his Mach I breakbarrel springer, which got over 1150 fps in .177 cal. 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 after leaving the muzzle makes no difference. 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.

Speed sells

However, the other side of the coin is that high velocity sells guns. A company that advertises its gun shoots 1000 fps and higher attracts lots of attention and, yes, sales. In fact, so much attention has been given to 1000 fps 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.

So why don't "they" make pellets that can go faster?

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 it brings all its bullet weaknesses with it. The first is that nobody can load a lead bullet directly 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.

So, no solid pellet currently on the market can be loaded into an airgun easily enough to use. If it could, the second problem would crop up. The twist rate of the rifling is too slow to stabilize a solid pellet. That twist rate, which is very often 1:16, 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 so they will spin faster to stabilize. However, they're designed to do just the opposite. Being both very heavy and also having a lot of friction with the bore, they go much slower in any given airgun.

Just make airguns more powerful

About seven years ago, I tested these solid pellets extensively in an AirForce Condor--the only air rifle that can get them up to 1000 fps. You know what? They still aren't accurate. They're stabilized at that speed, but they still shoot 5-inch groups at 50 yards, while 10 diabolos going less than 950 fps 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 or maybe increase the twist rate? That's the real question, and it's also the answer, in a way.

When an AirForce Condor shoots a 30-grain solid "pellet" at 1000 fps, 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 1000 fps and will reliably kill a woodchuck at 75 yards with ease, yet it still won't travel down-range 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 turned the Condor air rifle into a .22 short.

Let's stick with airguns

It's not the power of the gun at the muzzle, but how far down-range it throws the projectile that makes it more or less safe. The diabolo pellet has achieved something truly remarkable--a safer bullet. They allow it to shoot safely at shorter ranges, yet still have all the challenges of the firearm. While it is now possible to surpass this safety that has been in place for over a century, I don't believe we should. If we want the extra speed and range, a rimfire is the next logical step.
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Author:Gaylord, Tom
Publication:Shotgun News
Date:Oct 20, 2011
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