Why does a spiral or helical groove on the inside of a gun barrel cause the projectile to spin? I thought I understood it intuitively but then realized that I don’t really understand where the spin comes from; how it is generated.
Unless I’ve misunderstood the fundamentals of rifling, the usual approach is to somehow etch or impress thin helical depressions in a continuous manner on the inside of the barrel. The part of the barrel that remains “unetched” is called the ‘land’.
As the projectile travels down the barrel, I assume there’s more friction produced where it’s in contact with the land. As a result, I assume that those parts of the projectile in contact with the land will tend to move slowly than those which happen to be passing by a grove (where there should be no contact). A moment later, there will be changes in which parts of the projectile are then in contact, or not, with the land. Some of those parts which were were a moment ago in contact with the land and slowed by friction, are no longer - I assume they will tend to speed up. I don’t see how such a sequence promotes spinning of the projectile. In fact, I would have expected it to simply cause the projectile to lurch about or wobble. Where does the spin come from?
I must be missing something really obvious since everything I’ve read about rifling seems to just take it as a given. Can you educate me? It’s okay, I’m prepared to be embarrassed.
right. like runner pat said, the bullet is just slightly larger than the bore. the pressure from the burning propellant forces the bullet to swage down to the bore diameter, and the lands of the rifling cut matching grooves into the sides of the bullet, which forces the bullet to spin.
polygonal rifling doesn’t have sharp lands and grooves, but the end result is the same.
It takes a fair amount of force to swage the bullet into the rifling. Not a problem when you have powder doing the work, but it was (and remains) a problem with muzzle loading arms.
Ether a cloth patch was used as an interface between the ball and the rifling, or a hollow based projectile (Minie’ ball) were used, and the rifling was quite shallow and slow.(on the order of 1 turn in 75-100 or more inches)
The rifling was shallow so that it was easier to start, and slow because faster was more likely to ship when so shallow.
A round ball does not require spin for stability. The the minie’ ball being close in shape to a badminton shuttlecock requires very little.
With these projectiles the function of the spin is not to impart stability so much as to make aerodynamic imperfections impart a helical path rather than a curve in a single direction, keeping the point of impact far closer to the point of aim. Arrows and modern air rifles firing pellets operate on the same “error averaging” principal.
Modern muzzle loaders frequently employ soft plastic sabots, allowing deeper, faster(1:28 is typical) rifling that will stabilize longish, tail heavy bullets.
Back to breech loading arms:
In addition to swaging down of an over-bore projectile, soft enough lead projectiles can obturate when the powder ignites, allowing them to expand to fit the bore tightly. This requires matching of the lead alloy/temper to the powder charge. It is not effective with jacketed bullets. Obturation is what happens when you squeeze a marshmallow.
Ah. Thanks to all. I hadn’t realized that the bullet was actually a bit wider than the interior barrel measurement - that clears up a fair bit right there.