A lot of airgunners that hunt predators and larger game with mid-sized and big bores are moving to cast bullets. In the right guns, i.e. those that are designed to generate high energy levels and matched with barrels of the correct twist rate, these bullets will provide greater effective range and very good terminal performance. However, there is a class of larger-caliber air rifles that are designed specifically to shoot Diabolo pellets, and I think some very sound reasons for using them. It may be counterintuitive, but it is because a Diabolo pellet is less efficient that they are a better choice for some hunting applications. With a relatively poor ballistic coefficient these pellets shed velocity quickly and the configuration doesn’t penetrate as far as a cast bullet. I’ll come back to why these are useful characteristics, but first let’s take a look at the Diabolo pellets.
A Diabolo pellet is what most people think of when it comes to airguns. It is characterized by a flailed skirt (like a badminton birdie), a narrow waist and the head. The heads are usually sized to fit snugly into the barrel while the skirt is typically only lightly touching the rifling. The result is that most of the surface area of the pellet is not touching the barrel which in turns reduces friction. There are a number of different head configurations used on Diabolo pellets: round-nose (domed), hollowpoints, field points, hollowpoints with integrated polymer tips, wadcutters and a number of hybrids. For most hunting applications I generally use a round-nose design, though the polymer tips can be effective in the right situation.
The skirt is important in that it will obturate (expand) when the volume of compressed air hits it, with the result that the skirt engages the barrels rifling (along with the pellets head). This imparts spin on the projectile, resulting in stability and ultimately influencing the accuracy of the rifle-pellet combination. The second result of the flaring skirt is that it creates a tight seal between the pellet and the barrel, optimizing the efficient use of the small volume of air.
There are more calibers available today than ever before; the standard caliber .177, .22, .25 along with the niche .20 caliber have been around for several years. But for airgunners shooting anything over .25, there were only a couple of manufacturers of larger pellets, they were expensive, hard to find and didn’t work very well in high-powered guns. Over the last few years some of the major pellet manufacturers (JSB in particular) have started producing larger pellets in .30, .303, and .35 calibers and I hear rumors of a .45 in the works.
The Diabolo pellet is just about the perfect projectile in most standard-caliber guns, because they work well at subsonic velocities. When velocities get higher than that, or the energy is dramatically increased to generate higher velocities, a Diabolo pellet may destabilize or may even be deformed, compromising accuracy. This is one of the reasons many big bore shooters moved to solid cast bullets, even though the velocities are subsonic the larger volume of air and often higher fill pressures used do not play well with these pellets. Larger-caliber airguns also work more efficiently, and the powerful big bores used for hunting larger game utilize the heavier and more efficient bullet for this type of hunting. In addition, the bullet design has a much better ballistic coefficient than the Diabolo pellet, allowing the hunter to reach out much further.
So why not give up the Diabolo pellet and move to solid bullet designs in all my rifles? As mentioned for the lower-energy, standard-caliber guns, the reduced weight of the pellet and the reduced surface area engaging the rifling make this the optimal projectile in most standard guns in .177 to .25. There are a handful of custom .25s (.257 actually) built for long-range shooting that do in fact work better with cast bullets, however these are the exception rather than the rule.
On the other hand, I just said that powerful larger-caliber guns (the .30s on up) work better with solid bullets, so why would you want to use a Diabolo pellet in this caliber? It depends on what you are using the gun for and what the gun was designed to do. Over the last few years many mid-bore guns (.30, .303, .35) have come to market that are designed to work with Diabolo pellets and operate at energies in the 90-130 ft./lbs. range. Remember that with airguns, unlike firearms, the energy come from the gun not the bullet. A gun configured to produce power in the 200 ft./lbs. range will work well with a heavy cast bullet but will destabilize a pellet. A gun configured to generate 100 ft./lbs. will use the pellet effectively, but a heavy cast bullet will have the trajectory of a brick tossed underhand.
A logical follow-up question is why would a manufacturer design a gun in .30 or .35 that generates lower power. The advantage of these guns is precisely related to the characteristics of the Diabolo design. Because the pellets are lighter, less energy is required to accelerate them down and out the barrel at a high velocity. But due to the lightweight and relatively poor BC, they will shed that velocity/energy quickly, which reduces the distance they will carry. And at closer ranges on medium-sized quarry, the lighter projectile will dump that energy on target rather than passing through. So what you end up with is a gun generating the power of a .22 rimfire, producing a much larger wound channel and carrying a fraction of the distance. You essentially end up with a gun that inside of 60 yards will be a more effective gun for coyote, fox, bobcat. But more to the point, it is quiet and is safer because the projectile won’t travel much past the length of a football field.
I think that if you are a predator hunter that has an opportunity to hunt in more built-up areas (and these can support a lot of predator activity) you can appreciate the advantage of a gun that is quiet, accurate, powerful and safe to use on more space constrained settings.
So there you have my thoughts on the topic; my standard-caliber guns will continue to use Diabolo-style pellets. I mostly hunt with the round-nose configuration, but will use specialty pellets with some guns and for some applications. In my powerful mid- and big-bore guns I’ll stay with cast bullets when hunting big game or reaching further out.
But when using a gun in the .30s for predator hunting in urban/suburban areas, I think a Diabolo pellet is the way to go. Most of the pellets I use are from JSB, though H&N has been bringing out some really solid performers of late. Remember to match the pellet to your gun, and the gun/pellet combo to the hunting application it will be used for, and you’ll get great results!