Some thoughts on pellet design: Part 2
by Tom Gaylord, a.k.a. B.B. Pelletier
Today, we’ll look at my solution to the solid pellet for a 20 foot-pound rifle.
The first thing you have to know is how much this pellet will weigh. There’s a very quick and easy way to determine this. Find the weight of a round lead ball in .22 caliber. The .22-caliber Gamo Round Ball weighs 15.43 grains. When I did my design, I actually weighed round balls and found them to weigh 15.3 grains. So, that was my staring point.
Now, mentally extend the ball into a cylinder of the same width and height dimensions. The cylinder will be flat on top and on the bottom. The extra lead will increase the weight of the cylinder to just over 20 grains. Remember, this cylinder is just as long (0.2165″) as it is wide (0.2165″).
What velocity will a 20 foot-pound rifle drive a 20-grain pellet? Go to the Pyramyd Air velocity calculator and enter the pellet weight and energy…but you don’t even need to do that, do you? Because, if the energy is the same as the pellet weight in grains, the velocity has to be the magic number — 671 f.p.s. Try it in the calculator and see. Use the second formula for your calculations.
Is this precise? Not yet, because we don’t know the exact weight of the new pellet that hasn’t yet been designed. It’s just a ballpark approximation that gets you going. But even if your finished pellet weighs 24 grains, you still know that it has to be driven above 600 f.p.s.
I said I wasn’t going to ask you for the math, but I know many of you will find that easiest to envision, so here it is.
The formula for the volume of a cylinder is Π (pi) times the radius squared times the height of the cylinder. The formula for the volume of a sphere is 4/3 Π times the radius cubed. Lead weighs 0.4092 lbs. per cubic-inch.
I did the math and determined that a cylinder of lead that is 0.2165 inches tall by 0.2165 inches in diameter weighs 22.83 grains. Using the same math but a different formula, I calculated the weight of a lead sphere of 0.2165 inches diameter weighs 15.2 grains. That’s close enough to the 15.3 grains that I measured to use these data.
My estimates were not exact — they were close. Close enough to make this workable without doing the math because, at this point, what we’re doing is determining the envelope for the new pellet. Blog reader Mark was the first person to weigh in with the results. He said the pellet has to be short and fat. Bravo, Mark!
I asked you to design a solid pellet for a 20 foot-pound rifle because that is the hard task. What I actually did was design two pellets — one for 20 foot-pounds and one for 60 foot-pounds. What I’m about to show you are both pellet designs. The 60 foot-pound pellet has my concept thoughts on the drawing, while the 20 foot-pound pellet has just the dimensions.
20 foot-pound solid pellet design
As you can see, this design is lighter than the cylinder of lead described above. So, it goes faster and spins faster, thus having a greater chance of stabilization. This one probably weighs 19-20 grains.
Also note that the driving band at the rear is very thin, so loading should be easy. The body of the pellet is smaller than the smallest diameter of the target bore size of 0.2165 inches.
Notice that the band at the head doesn’t get engraved by the rifling. It rides on top of the lands. The rear band is what seals the compressed air behind this pellet. This also helps the loading.
Notice that this pellet is shorter than it is wide. It’s very stubby, which we believe to be necessary for stabilization in a lower-powered air rifle.
Will it work?
In theory, yes. But until you make some pellets and test them, you won’t know. It may work well in some guns but not in others.
60 foot-pound solid pellet design
The 60 foot-pound pellet is different than the 20 foot-pound pellet. The basic idea remains the same, but there are small yet important differences to work with the 3x higher power.
I gave no dimensions with the 60 foot-pound pellet because I wanted to test the 20 foot-pound pellet first. Until we knew how well it performed, the larger pellet was just a concept.
These pellets were designed to work well in the pneumatic rifles mentioned above. You’re looking at the exact design drawings that were used to get the solid pellet project underway.
My pellet was never made, but there is a similar pellet on the market that came out a year after I shared these drawings with the company. That pellet is made by a different company. It has some differences from my design; but if you look at it, you can see the similarities. That pellet is still in use today and seems to work as designed. So, I feel confident that my design would work.
This is as much of the story as I’m willing to tell.