Dynamic Stability

[R_A_W]

I posted this in another thread in response to @Frank Green , but it was entirely tangential to the topic... @Ledzep @Bryan Litz Ballistics perhaps y'all have some insight as well.

When you see a pitched bullet & the counter pitching force is sufficient to correct after exiting from the bore, does this correct without any angular deviation to the flight path?

E.g., assume a bullet is seated measurably eccentric and is fired, engraving eccentrically. It will obviously initially spiral after exiting the bore. Assuming dynamic stability is achieved, will the projectile's subsequent flight path have discernible angular difference in flight from a projectile that exited the bore perfectly with no pitching? Is the axis or locus of correction the center of mass? I'm guessing no, due to air resistance.

Hopefully that made sense. I'm guessing there are too many factors for a simple answer, like bullet shape, length, weight, speed of rotation, velocity, degree of wobble, etc.

 

[Ledzep]

If you can clock the eccentricity to the same angle and to the same magnitude it will produce surprisingly consistent results. Those impacts, however, will be deviated from non-eccentric shots.

If you don't clock them to the same orientation but have the same magnitude you will get a donut group. If you clock them the same but don't control magnitude they will string impacts.

Generally poor dispersion is a result of variation in magnitude and clocking of those eccentricities. Be it eccentric mass or in-bore yaw.

ETA: Muzzle exit gas flow can also play a part, especially if you're on the edge of dynamic stability to begin with.

 

[Frank Green]

I'm pretty sure I know what your asking but not sure if there is an exact answer.

I will say the flight of the bullet for sure is effected...but how much? To many variables to compute that. Like how much did the bullet start off center if if you will eccentric. The bullet is basically starting with a premature wobble. Ammo loaded like this and or poor quality bullets and accuracy suffers. A faster twist some think will help offset this but in my experience it makes it worse.

Accuracy testing was done in late 2022. They found that at the 80k psi pressure area....bullets fired out of the conventional rifling which was 6 groove the bullets got a burr on the side of the jacket. That burr was as big as a .032". I seen the pictures of the bullets. High speed photography right after the bullet left the bore. That burr reflected in every camera flash. You could see it like a sore thumb. Every bullet fired out of the 6 groove barrels did it. There were 7 barrels. But.... the bullets fired out of the 5R rifling (there where five of those barrels)....not one bullet had the burr. Accuracy difference was only .100" at 200 yards. The 5R barrels out shot the 6 groove. Yes a .100" isn't a hill of beans. All 10 shot groups. A total of 320 rounds fired.

So next question I asked was... did it effect the bc of the bullet. They said yes.... as much as 5%. Doesn't sound like much but factor in that 5% at 500 yards, 1000 yards..... etc... and your groups will get bigger or should I say have more of a variance and shots wondering etc...

 

[R_A_W]

Thank you both, great answers.

I would imagine eccentric projectile mass is less common than projectile runout from seating with modern, higher quality mass produced ammunition. Am I wrong here? However, slightly eccentric mass should generally be much more problematic downrange than the degree of in-bore yaw that could be expected to result from factory ammunition with runout (barring other variables like jacket damage)?

I realize that's kind of an apples to oranges, particularly without any magnitudes.

 

[LRRPF52]

SixFive Guys did a 100rd sample of each to compare, where one batch of ammo was loaded with a single stroke during bullet seating, and another was double-seated with an initial partial-depth seat, then the cartridge rotated quarter turn, and fully-seated to shrink runout.

The double-seat cartridges shot better groups than the single-seated batch because concentricity was better.

 

[Macht]

I realize that's kind of an apples to oranges, particularly without any magnitudes.

I agree with your last statement, magnitudes are important. Not only important, but difficult to obtain. I'm unsure how many bullet manufacturers (if any) use a spin balance to measure mass eccentricity. Cartridge runout is easy to measure but doesn't necessarily correspond one-to-one with the degree of asymmetric engraving.

The two affect the trajectory in different ways. Eccentric mass alone gives purely a lateral throw-off effect, where in-bore tilt causes both lateral throw-off and aerodynamic jump.

I don't think it's possible to answer your question without actual numbers.