Why Bullets are Spun for Stability

[Kentucky Wind]

After several years of this thread being started, I find interesting to reply.

It DOES matter the direction of the bullet spin it, however, does not affect the aerodynamics.
The reason it DOES matter the direction of the torque applied to the bullet, has to do with the resulting vector inherent to an free-flying object.
The bullet spins clock-wise so the torque applied to a free-flying object results in a deviation of the flight path, in this case to the right.
You can of course see this effect in LONG distance shots, which have a lot of other factors that modify the bullet path.
Snipers compensate for this effect, as for gravity, using Kentucky Windage, which is the easy way for the "click" compensation of sights.

 

[Ranger Mike]

Cliff_J,
A click is used in that way is military jargon for 1 kilometer. The Army seems to like clicks very much because everywhere we went on foot was always several clicks away.

The meaning of a click in shooting has to do with the sight, or scope on a rifle or pistol. As the sight is adjusted, it "clicks". Each click represents a given amount of elevation or windage at a given range. One click on a military rifles often corresponds to one inch at 100 meters(used to be Yards).

actually a click , more like several to be exact(for the 11B gravel agitators) is that nebulous point on a map some staff puke decides to send you...,,where just before you arrive ( usually at night carrying 90 pounds of toys) you are then notified that the mission is call off and you do an about face and return to base by another route,,,

 

[Mr_Firelock]

In terms of a bullet's trajectory and why it spins, the secret lies in the interior of the bore - it has a spiralling cut through it.

If you ever look at a cross section of a barrel, you will see a series of spiralling cuts inside the bore. This is called rifling (where the term Rifle comes from.) Basically you make the windage (the gap between bullet and bore inside the barrel) as small as possible to focus the bullet's trajectory in a certain direction, and the bullet will engage itself on the spiralling cuts, causing it to spin. By spinning, it practically drills through the air, allowing it to break the sound barrier easier, accelerate faster, cut through the air easier. The result is that once it exits the muzzle it is as fast as possible. Furthermore, by cutting through the air easier, it travels a better trajectory and travels faster (thus more accurate and straighter, and higher penetrating,) and further (due to increased velocity.)

Prior to rifling becoming widespread (though it has existed since the 1500's) all firearms were smoothbore - there is no rifling inside the bore and it's pretty much just a tube. Whilst it was cheap to make and quick to load, firearms were inaccurate. Rifles did exist, though they weren't used by the military extensively for many centuries to come because rifles were expensive to make and slow to load (this is the time of muzzle loaders were you load down the barrel of the gun, and only get one shot per barrel, Rifles were hard to load because the windage is very small and hard to ram a ball down into the powder charge.) If you've ever seen movies like, "The Patriot," there is a scene where the Continental Army and Redcoats engage each other out in a field - they march in shoulder to shoulder to close range, then start firing. This is how armies fought each other from the 1500's till about the early 1800's, because their smoothbore muskets were only accurate to ranges of about 50-70m and they would all fire together at once, to increase their chances of hitting a target.

This idea worked for hundreds of years, but Rifles still saw occasional use. A good example is in the American Revolutionary War in which American Minutemen would use their Rifles (which could be accurate out to 200-300m compared to Smoothbore Musket's 50-70m) to just shoot the Redcoats before they were in range, then run away and reload to repeat again later on. Once Rifling became able to machined easily rather than requiring skilled gunsmiths (which were few) they began to see more common use, until finally in the mid 1800's the Smoothbore Musket disappeared in favour of the Rifle.

Rifles proved more accurate, longer ranged and more deadly. The ability to make a projectile spin is why they are so effective.

 

[M James]

I don't think that's completely accurate; the bullet doesn't 'screw itself into the air' because it is smooth. The spin imparted by the rifling gives the bullet gyroscopic stability, it is this that improves the range and accuracy over an un-spun projectile.

If you imagine a bullet in flight, that isn't spinning, the leading edge of the round will have slight abnormalities on one side relative to the opposite side, there may also be differences in air pressure on either side of the bullet. Due to the extreme speed of the round, these minute differences conspire to rotate the round (around it's pitch and/or yaw axes if you like) in a particular direction, until it is no longer stable and begins to tumble. By spinning the bullet, these forces act in a spiral, thus cancelling themselves out.

Also, for a given charge, the muzzle velocity would be lower for a rifled barrel because some of the force used to propel the round is used up in making the bullet spin.

Also also, someone mentioned (a few years ago, *cough*) that the spin of the bullet has nothing to do with the damage caused on impact. Soft and hollow point rounds are designed to mushroom on impact, imparting more or all of their energy in a shorter distance. If this is coupled with a high enough spin rate the round may fragment as it deforms.



That's my take on it anyway!


I was wondering if anybody knew if the rifling's twist rate was constant along the barrel length, or if it got tighter towards the end?

I'm trying to work out if it's possible to induce a spin magnetically, thus reducing the charge needed for a given muzzle velocity and reducing the mechanical stress on the barrel. I've seen a couple of posts here and there about it, but my googlefu can't find much recent discussion about it.

 

[nsaspook]

Also also, someone mentioned (a few years ago, *cough*) that the spin of the bullet has nothing to do with the damage caused on impact. Soft and hollow point rounds are designed to mushroom on impact, imparting more or all of their energy in a shorter distance. If this is coupled with a high enough spin rate the round may fragment as it deforms.

Bullet ballistic gelatin block videos. http://www.brassfetcher.com/index_files/HighSpeedVideoSlowMotion.htm

50 BMG ballistic gelatin block video http://www.youtube.com/watch?v=xYSGuiko6Gg&feature=related

Most of the damage is caused by the shock-wave from the deceleration of the bullet in the media. Long thin bullets (The .223 FMJ is a good example) tend to flip and break causing a massive cavity from a small but high speed bullet. I would always chose a M14 (.308) over a M16/M4 in most cases during a security watch because it has more stopping power at long range with much better ballistics (second shot on target) during bad weather conditions.


http://upload.wikimedia.org/wikiped..._protection_watch_on_board_USS_Blue_Ridge.jpg