Pellet head sizes
by B.B. Pelletier
Late last week we received this question: Pyramyd lists 2 JSB Exacts in .22, 5.51mm and 5.52mm. Why? High power gun vs low power gun?
Today, I'd like to discuss the reason for different pellet head sizes.
What does a pellet head do?
A diabolo pellet only touches the bore in two spots - at the head and again at the tail. The tail is sized much larger than bore size to seal the compressed air or gas behind the pellet. But, the nose doesn't seal anything. It acts as a guide for the pellet. It can either ride the bore, which is the top of the rifling lands, or it can ride the grooves, themselves. If a pellet is marked by rifling on the nose when run through the barrel with a rod, it is riding the grooves, which is the most common way.
This illustration of a typical diabolo pellet shows how the head and skirt touch the bore, while the rest of the surface does not. This is a low-friction projectile. The flare at the base of the skirt has been exaggerated for this illustration.
Pellets exert less friction than bullets because they touch only the barrel in these two areas. They can get away with so little contact because the gas or compressed air that pushes them is at much lower pressure than the gasses created by burning gunpowder.
Pellet skirts are sized by the breech.
When a pellet is loaded into the barrel, the skirt is sized down to the bore size as it is pushed into the breech. If you examine the skirt of any diabolo, you'll notice that it is thinnest at the very end. Many pellets even have sloped walls at the ends of their skirts to further reduce the amount of lead that has to be squeezed down. Not only is the skirt being sized at loading, the rifling is also cutting notches in the side of the pellet - usually at both the head and the skirt. All of this causes resistance, making it harder to push the pellet into the bore.
Beeman Kodiak at left has rifling marks on both the nose and the skirt. The skirt is flattened where it was sized by entering the breech. Solid "pellet" at right had to be totally engraved by the rifling - it was MUCH harder to load.
Solid pellets load hard!
Those of you who have shot the so-called "solid pellets," which are actually bullets in disguise, know how hard it is to get them started in the bore! That's because the rifling has to engrave the entire length of the pellet - not just the head and skirt of a diabolo. What you are actually doing is muzzleloading your gun at the breech and using your thumb instead of a short starter to start the pellet in the bore. Ouch!
What do you do with pellets that come in different nose sizes?
You shoot them in your airgun to see which size is more accurate. The size has nothing to do with the gun's power level. Most likely, one size head will outshoot all the others in a given gun, though you may have to test at long ranges, beyond 30 yards. When you find a head size that works, chances are the same head size will work with every kind of pellet in that gun. To see an example of pellets sold by head size, go to the JSB Exact pellets and look at the 5.51mm and 5.52mm head sizes.
Head sizes are most popular and the most common in target pellets. Ten-meter target shooters go through a lot to determine which head size and even which lot number of pellet is best in their guns, then they buy 30,000 to 100,000 pellets at one time. It's not unusual for them to go through 30,000 pellets in a single year. If you look at the bottom of a tin of high-quality target pellets, chances are good it will have a sticker with the head size.
Does size matter?
It matters to those who are in search of ultimate accuracy for a given airgun. If thousandths of an inch matter, then head sizes do too. For most of us - especially those shooting sporting airguns, head size issue is unimportant.