Pellets' fit in the bore: Part 1
by B.B. Pelletier
An anonymous reader asked how to determine if a pellet fits well in the bore of an air rifle, and also if there's a lot of air blowby. Today, I'll discuss this and also, for those who can watch videos, I'll point you to a very revealing website where you can watch high-speed photography of what I am talking about.
Do you think that a pellet seals the bore so no air gets past? It sounds nice, but it doesn't happen. Air does get past (blow by) all pellets when they are fired. The best situation is for the least amount of air to get past, and several things contribute to that. They are pellet skirt diameter, skirt wall thickness and the hardness of the lead. We will look at each in turn.
You might think that if a pellet is larger than the diameter of the bore, no air could get past it. But some always does! I reckon it blows past at the instant the gun is fired. In the time it takes for the pellet to go from stationary to moving, air is rushing past - going through any available passage. But if the pellet is larger than the bore, what passages can there be? Well, for starters, the rifling distorts the pellet in many places, creating small openings. Air can rush through those tiny openings that form at the junction between the smooth part of the bore and the point where each land starts to rise.
Where the lead is forced to conform to the rifling lands, tiny openings form at the junctures where the lands start rising. This drawing isn't to scale, but air doesn't need much of an opening to escape.
Lead is the most malleable and affordable material from which to make pellets, so even though these small imperfections occur, there really isn't anything better at doing what lead does.
Here's the proof
For those with computers that can run videos, the proof of this is shown here. At the top of the page choose English. Then choose examples. On the left side of the page, choose videos. On the page this takes you to, click on the third video from the top, which is a .45 ACP exiting the muzzle at 70,000 frames per second. When you select that video, it will download to your desktop as a zip file that you must then click on to unzip and see. (A lot of work to see a video!) The unzipped file size is just 3.5 MB, so the download is quick if your internet service is broadband. My thanks to the anonymous reader who gave me this link.
For the rest of you, here is what they'll see. When the .45 automatic fires, the first thing that exits the muzzle is gas from the burning powder. Then the bullet comes out, followed by a denser cloud of gas. This is dramatic proof that gas does blow by, even when the bullet fits the bore exactly.
Skirt thickness and hardness of lead
Thin pellet skirts tend to be blown out into the walls of the bore by the sudden blast of air. This is not significant in PCPs, but it is in spring-piston guns, where the air blast is both sudden and violent. PCPs use more air than springers, but they release it over a longer time, so the push is more gradual.
Thin-skirted pellets that are also made from soft lead deform the most. When I shoot the Hakim .22, which is a taploader, I always use RWS Superpoints, which have the thinnest, softest skits I have found in a premium pellet. Even though the Hakim isn't powerful, it seems to blow out the skirt of the Superpoint to fill the loading tap, and I get more energy with them than a lot of other pellets.
RWS Superpoint has a thin, soft, lead skirt that lends itself to distortion.
Tomorrow, I'll finish this discussion, unless your comments give me more to consider.