Some thoughts on pellet design: Part 1
by Tom Gaylord, a.k.a. B.B. Pelletier
Blog reader Kevin asked me to tell “the rest of the story” about a pellet I designed a few years ago. That design never got to market under my name, but it did appear on the market from another manufacturer. How that came to be is a question that I can’t answer, but my suspicions caused me to break my relations with the company I’d been working with.
I’m not naming names because I cannot prove what happened beyond the obvious parts that affected me. But, today, I thought I would share some of my ideas about pellet design — ideas that have been proven because they’re now in production and are apparently successful.
Not a novice
Usually, I would make all sorts of self-depricating remarks at this point about how I’m not qualified to design pellets, but this time I don’t think I have to. The things I’m about to tell you are things I’ve observed in my experiences with airguns, and I know them to be true. Sometimes, you can learn a lot just by doing things.
When I was working for AirForce Airguns, the owner, John McCaslin, asked me to test a couple heavyweight pellets in the Condor rifle to see how good they might be. The Condor is so powerful that we figured it could easily handle the heaviest pellets on the market.
The first pellets he gave me were a sample lot from a UK manufacturer. They were shaped like cylinders with points on either end, and the maker claimed they could not be loaded backwards because they were the same on both ends. When that’s the selling feature for something, you know it’s bad before you start the test, and I wasn’t wrong about these! They refused to enter the bore!
The “maker” had assumed that pellets must be bore-sized to seal the compressed air behind them. That part he got right, but he had obviously never loaded a muzzleloading rifle. Bore-sized bullets (they ARE NOT pellets, no matter how much anyone wants them to be) will not go into a rifled barrel without a LOT of force being applied to them — as in a hammer!
Had the maker known anything about the evolution of the rifle, he would have stumbled across the patched ball, which is considered to be the biggest advance in rifled guns since the invention of rifling. With a patched ball, the patch takes the rifling, leaving the lead ball unmarked. It is an order of magnitude easier to push a tight cloth patch into rifling than it is to engrave a lead bullet with the rifling lands!
The patch fills the rifling, sealing the gases behind the ball and causing it to spin without actually being engraved by the rifling. This advance made loading a rifle much easier and faster.
Well, the pellets I was trying to test could not be loaded into the barrel. But I forced them in with a hammer and punch, and the results were not good. Accuracy was terrible and velocity was in the 700s for this 30-grain bullet, when it should have gone about 1,000 f.p.s.
I then tested other heavy solid “pellets” for AirForce, but each of them had the same drawbacks that the first one did. They didn’t load easily.
I could regale you with dozens of other anecdotes involving solid lead bullets (that were invariably called pellets), but they all worked the same. I remember working with the original Pelletman, who found it hard to believe that the diameter of the bore of our Lothar Walther barrels was as tight as it is. He finally gave up when his 0.218-inch pellet proved too large to easily enter the breech of a Condor. Swaging dies cost hundreds of dollars apiece, and there’s only so much money to be lost on trial-and-error tests before you have to throw in the towel.
Lesson No. 1
A pellet has to be easy to load. If it isn’t, no one will buy it because they find it too difficult to use.
Spin or drag — what stabilizes a pellet?
We just started looking at this, using the vintage Diana 25 smoothbore. As that test seems to have revealed, both spin and drag are important. Drag for the first 10 meters of flight and spin after that — or at least that’s what the test appears to have demonstrated. Only powerful rifles like the Condor can drive a heavy .22-caliber pellet to supersonic velocities. The rest of them should probably also generate high drag. That rules out boattail shapes and solid bullets — even though you will find the airgun chat forums ablaze with thoughts of using them. At the velocities we’re able to achieve, the boattail shape is useless; and we still need a lot of drag, no matter how fast we get the pellet moving.
The .30-caliber flat-base bullet on the left is much shorter than the boattailed hollowpoint .30-caliber bullet on the right. The shorter bullet will stabilize with a slower spin than the boattail. The boattail will remain supersonic longer than a flat-based bullet of identical weight. But neither of these bullets has very much drag.
One thing I know for sure is that Winchester rifled their target rifle barrels, which were chambered for the .22 short cartridge, with a 1:22″ twist. That’s been documented. The .22 short cartridge fires a domed-shaped solid lead bullet weighing 29 grains. That happens to be very similar to the weight and composition of a heavyweight .22 airgun pellet. A standard-speed .22 short bullet usually leaves the muzzle at 1050-1,100 f.p.s.
I also know that the shorter a solid conical bullet is, the slower it has to spin to stabilize. Short bullets stabilize sooner than longer bullets when fired from the same barrel. I learned that on the rifle range, but it’s also very evident in the literature.
Lesson No. 2
A shorter solid bullet is stabilized with less spin than a longer bullet of the same weight.
Okay, so given lessons one and two, how would you design a solid .22-caliber pellet that you wanted to be extra-heavy? That was what I was trying to do when I had the problem I mentioned at the beginning of this report. I wanted heavy .22-caliber pellets that would perform well in PCP rifles generating 20 foot-pounds, 35 foot-pounds and 50+ foot-pounds. My thinking was to sell these designs as a brand but also approach the makers of some powerful PCP rifles and see if they wanted to buy the design. We would make those pellets only for them. That’s a business strategy.
Let’s concentrate on the 20 foot-pound pellet for now, because that one will be the most difficult to make. Because it will be going so slow — wait a minute — do you know how slow it will be going?
I want you to take the design from this point. The tools you need are available on the Pyramyd Air website. This drill takes almost zero mathematics — but you do have to use the tools that are available and you have to think the problem through.
Design a solid pellet that should work well in a PCP capable of generating at least 20 foot-pounds of muzzle energy. You don’t have to draw your pellet for us, but you do have to describe what it looks like and why you designed it that way. And tell us about the expected velocity this solid pellet will have to travel.
On Monday, I’ll show you my design and tell you why I made it the way I did.
This doesn’t have to be the last time we look at pellet design. I have a LOT more material I can bring into this discussion if there’s interest