Epicyclic Swerve

[ADMIN]

This work was done to investigate a question about the grouping ability of rifles at various ranges. Many shooters, including myself, have observed the strange phenomenon of a rifle that groups angularly smaller at long range than short range. In other words, a rifle that groups 1" at 100 yards grouping 1.5" at 200 yards (you would expect no less than 2" at 200 yards). There are abundant theories about why, how, or even if this phenomenon actually happens at all. The scope of this paper is to focus in on one mechanism that's been used to explain this phenomenon: the mechanism of epicyclic swerve. Epicyclic swerve is the technical term for the corkscrew path that a bullet flies as its nose precesses around the flight path. Some people think that epicyclic swerve explains the observed phenomenon of angularly smaller groups at longer ranges. Read More...

This is a thread for discussion of the article, Epicyclic Swerve, By Bryan Litz. Here you can ask questions or make comments about the article.

 

[charlieprecision]

Bryan,

Your analysis here pretty much shows this phenomenon can't be caused by epicyclic swerve.

A while back, in another post, you stated (words to the effect) that the smaller group at longer range phenom may be related to sight picture and target distance. Last fall I accidentally discovered how amazingly sensitive point of impact is to very small variations in point of aim. (There's some interesting physics there, I'll bet.) I was shooting at a target with a fairly large aiming shape, and I could not get groups smaller than 1 MOA, where I had shot tighter groups before. Finally I switched to a target with a small red dot, where my crosshair thickness was only about 1/3 of the dot diameter at 100 m. All of a sudden I could shoot 1/3 to 1/2 MOA with the same rifle, ammo, and support setup (prone, bipod, rear bag). The reason was that I could now see the tiny aiming variations, where before, my crosshairs were adrift in a much larger visual space.

Here's my theory: If someone uses a relatively large target aiming point size, moving the same target to longer range will cut the angular size of the aiming point relative to crosshair thickness, making it easier to see the tiny aiming variations that must be controlled for precision shooting. Given the right conditions, the result will be a smaller group at longer range. I think the best situation is when the aiming dot is only slightly larger than the crosshair thickness. Then you can see incredibly small aim point variations.

 

[HighDesertHunter]

After reading the article I started thinking about how one would model interior ballistics. Ideally to capture normal variability we hand loaders put into our ammo such as neck tension, concentricity, ogive to land distance, ...

It would be nice to have a model to focus reloading efforts on what really matters. Not to mention, you could simulate "real" input boundary conditions for you exterior ballistics model as well.

Since I have just started thinking about the topic, I don't have any ideas as how to do that but I will start doing some literature search on the subject and see what I can find. I am curious if you already have this type of model...?

My guess is that there is some existing empirical muzzle exit data that might be easier to use for boundary conditions but a controlled experimental set on load variability I have not seen.

 

[BergerFan222]

Interior ballistics is a very challenging modeling problem.