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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.

diabolo-web
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.

author avatar
B.B. Pelletier
Tom Gaylord is known as The Godfather of Airguns™ and has been an airgunner for over a half-century, but it was the Beeman company in the 1970s that awoke a serious interest in airguns. Until then, all he knew were the inexpensive American airguns. Through the pages of the Beeman catalog, he learned about adult airguns for the first time. In 1994, Tom started The Airgun Letter with his wife, Edith. This monthly newsletter was designed to bring serious reports about airguns to the American public. The newsletter and Airgun Revue, a sister magazine about collectible airguns, was published from 1994 until 2002, when Tom started Airgun Illustrated -- the first American newsstand magazine about airguns. Tom worked for three years as technical director at AirForce Airguns, the makers of the Talon, Condor, and Escape precharged air rifles. Today, he writes about airguns and firearms for various publications and websites. He also makes videos, and you'll find short clips embedded in some of his artices on Pyramyd AIR's website. Tom is a consultant to Pyramyd AIR and writes under the name of B.B. Pelletier.

2 thoughts on “Pellet head sizes”

  1. I mesured head sizes for about 20 different pellets, both .177 and .22. In .177 they varied between .168 and .174. In .22 they varied between .211 and .2135.

    The Chinese pellets had the most variable in their respective container. The better pellets had the least variable which was only about .001.

    Question is, should the pellets with the closest measurement to the desired caliber be the choice?

    Of course the pellet with the least change of measurement in it’s container would be preferred.

    I measured 10 pellets from each container for the average I obtained.

    Would it be a good practice to stick with the closer specs with the least variable?

    Thank You, F Nash

  2. F Nash,

    That’s a good question. The only way to tell for sure is to shoot several groups with each pellet in a given gun. That will tell which is best FOR THAT GUN.

    I would guess that the pellets with the least size varience would do better, but you can’t know for sure without shooting.

    Also, since barrels vary too, you’ll never know which pellet is closest to the bore size without measuring both the bore in question and the pellets. And even then, you still won’t know whether the closest pellet is the most accurate until you shoot them.

    Shooting turns out to be the best way to select good pellets.

    B.B.

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