What do pellet head sizes mean?
by B.B. Pelletier
This question came in last week in the form of a comment about pellet sizing. Pellets are sized by pushing them through a die, and it used to be popular to do it in the 1970s. Shooters eventually realized that the bores of their airguns were doing the same thing, and sizing wasn’t really necessary.
Pellet head size is a different topic that’s still very relevant, so today we’re going to look at what’s involved. It all begins in the past, when conical bullets were first used in firearms in the 19th century.
Conical bullets are longer than they are wide, so they weigh more than the round balls that served as bullets for several hundred years. But because they’re longer, they can also create more friction with the bore. When they’re loaded from the muzzle, this is a problem because it takes so much more force to seat them down on the powder that the effort will usually distort their noses, adding nothing to accuracy.
One early solution (ca. 1835) was the picket bullet, which is also called the sugar-loaf bullet due to its shape. It contacted the bore only at a narrow band near the base of the bullet. The rest of the side of the bullet did not touch the bore and, therefore, did not create any friction. Picket bullets were far more accurate at long range than round balls of the day. They pushed the maximum distance for accuracy from about 200 yards out to 330 yards. And they were also heavier in a given caliber, so they were deadlier on game in calibers that were too small for round balls. A .38-caliber round ball might be marginal for whitetail deer, but a .38-caliber picket bullet would be fine.
The picket bullet looks like a large piece of candy corn. It was a big advance from the round lead ball. Image from Ned H. Robert’s book, “The Muzzle-Loading Cap Lock Rifle.”
But there was a problem. If you tilted the picket bullet during loading — something that was all too easy to do, it left the bore in a tilted position, allowing gas to escape unevenly at the muzzle. That caused it to fly wide of its mark. Picket bullets were considered very accurate but also very difficult to load correctly.
You can see how an incorrectly loaded picket bullet would cause inaccuracy. Image from Ned H. Robert’s book, “The Muzzle-Loading Cap Lock Rifle.”
Straight-sided conical bullets were tried next, but shooters quickly discovered all the problems associated with friction that are mentioned above. Something was needed that would seal the bore against gas loss yet not create excessive friction at loading. Remember, too, that barrel makers were tapering their bores at this time, so that did help a little — but it wasn’t enough.
The next innovation was a bullet that fit the bore at its base, but rode on top of the lands for most of its length. A bullet with two diameters! This bullet worked very well and because it did, it increased the distance at which conical bullets were accurate from about 30 rods (330 yards) out to 1,000 yards.
The two-diameter bullet has a nose that rides on top of the rifling. If it’s sized correctly, this can be a very accurate bullet. Image from the “Lyman Cast Bullet Handbook, 3rd Edition.”
By the time breechloading rifles started coming into popular use (around 1850), the issues of bullet friction with the bore ceased being such a problem, so straight conicals continued to be used, as did the new bullets with bore-riding noses. The picket bullet even remained in limited used through about 1880 for handmade muzzleloading rifles.
A typical conical bullet has parallel sides and a short nose. Image from the “Lyman Cast Bullet Handbook, 3rd Edition.”
Black powder has one characteristic that smokeless powder lacks. When it ignites, it does so instantly, imparting a hammer-blow to the base of the bullet in front of it. Shooters of this era counted on the obturation — a blockage caused by expansion of the bullet bases to help seal the bore.
In the late 1800s, a world champion marksman, Dr. Walter G. Hudson, designed a bullet for breechloading rifles that mimicked the properties of a conical bullet in a black powder arm. Dr. Hudson, who was also one of the men who invented the gas check for bullets, designed a bullet of two diameters. The rear bands of the bullet were exactly bore-sized, while the remainder of the bullet was sized exactly to ride on top of the rifling without being engraved. This bullet didn’t tilt while it was in the bore if it was correctly sized.
Dr. Hudson shot his best matches with the new smokeless powder that didn’t obturate the base of the bullet. So his bullet went into the bore already obturated! It was an advancement that would have had far-reaching consequences if it hadn’t been replaced almost immediately by the new smaller-caliber jacketed bullets that went even farther and were more accurate at long range.
This kind of bullet must be handmade for each gun based on precise bore measurements, plus it cannot be loaded into a cartridge because the base bands are too large to allow the cartridge to be loaded into the chamber. The bullet must be loaded into the bore first, followed by a charged cartridge case that has no bullet.
Dr. Hudson’s bullet has two driving bands at its base that fill the bore of the gun. The forward bands ride on top of the lands. Though it’s difficult to see, the base bands measure 0.383 inches, while the forward bands measure 0.376 inches.
You may wonder what the difference is between the Hudson bullet and the two-diameter bullet. The Hudson bullet has bands and grease grooves that ride the rifling. The two-diameter bullet rides the rifling with a solid nose and no bands.
Pellets and their heads
Which brings us to the subject of diabolo pellets and their head sizes. In target guns, it’s considered ideal if the pellet is sealed by the skirt, which also takes the rifling, while the head rides either on top of the lands or is just barely engraved by them. It’s this “just barely engraved” phrase that causes target shooters the greatest concern. How much is enough? There’s really no way of knowing except by shooting the pellet and seeing the results.
Impossible to make every barrel the same size
Some shooters think that because barrels are made by button-rifling or by hammer-forging they can all be made identical, but they can’t. It’s impossible! Every barrel will respond differently to the passage of the rifling button. While the differences are minute, perhaps in the range of ten-thousandths of an inch, they’re still there and they do affect how the barrel turns out.
The goal for target shooters is to find pellets that group the best in their guns. We believe that these will be pellets that are either lightly engraved on their heads or perhaps are not engraved at all, but they do ride on top of the rifling with no tolerance. From the picket bullet discussion above, you can now see how tilting inside the bore would be a bad thing.
All pellets come with certain sized heads, and all true target pellets (not just those with the name “target” in their title, but pellets that are really controlled during manufacture) have their head sizes printed somewhere on their tin. It will be on a paper label most of the time — and you might have to look hard for it — but if it’s a real target pellet, the head size is there.
The paper labels on the back of two target pellet tins show the head sizes. In Europe, a comma is usually used in place of a decimal that we use in the U.S.
…you can now see how tilting inside the bore would be a bad thing.
Don’t stop yet!
If barrel makers cannot make every barrel the same, what does that say about pellet manufacturers that make tens of millions of pellets every year? That’s right, their pellets do have manufacturing differences. Even though it says a certain head size on the label, there’s no guarantee that every pellet inside the container is exactly the same. And, in the case of certain sloppy manufacturers, the head size might not even agree with what’s on the label.
Airgunners who are target shooters are fanatical about selecting their pellets. They try all the known good brands. If they have a good sense of what head size their gun likes, they might restrict their search to just that size; but often they’ll go both larger and smaller, to make sure they’ve checked everything. When they find the best pellet, they buy 25,000 or more, which will get them through a season of competition and practice.
Because this is such a chore, target shooters will often buy different lots of good pellets of different head sizes and trade them with other shooters. When they find the one best pellet for their gun, they then sell or trade the rest of the pellets they bought and buy a huge lot of the one right pellet.
Head size matters — sometimes
So pellet head sizes do matter — sometimes. But not for everyone. A guy who shoots an RWS Diana 34P for recreation would be wasting his time going through all the trouble I’ve outlined here. He only needs to find a pellet that works, and then shoot just that one pellet. And for most of us, that’s all you need to be concerned with.
When I test airguns for you, I usually don’t even bother to look at the head size. Indeed, a great many sporting pellets don’t even have their head sizes indicated. You can assume that it’s about 4.50mm, because that’s the size that seems to work the best for most guns. And unless you start shooting in competition or at very long ranges, that’s all you have to be concerned with.