Rifle barrel twist - can someone explain?

[Scallywag]

OK, I have another rifle-related novice question, and this site has become my #1 place to ask these kind of stupid questions and get educated. So here goes:

Can anyone explain to me the importance of the rate of twist in a firearm barrel, and why it's different for different calibres? I get the importance of rifling, but the twist rate is baffling to me. I guess the barrel length is related to this, too?

What would happen if you had the 'wrong' rate of twist in a barrel for the ammunition you were using?

Can someone explain the principle behind it, as dumbed down as you possibly can?

Thanks as always.

[Montell C. Williams]

The rate of twist controls how fast the bullet spins, the shorter the bullet in relation to its diameter the less twist it needs to be stabilized, usually a round ball has a rate of 1 turn in 66 inches, a conical like a T/C maxi ball would be best stabilized with about 1 turn in 35 inches and so on, thr rate of twist is usually a compromise so various weights of bullets can be used with reasonable accuracy.

Remember the longer the bullet in relation to its diameter the faster the twist needed.

[PisnNapalm]

The wrong twist rate can actually make the bullet unstable and send it off target. There is a balance between the weight, spin and forward velocity of the bullet that is necessary for accuracy.

http://en.wikipedia.org/wiki/Rifling
http://www.gunnersden.com/index.htm....ist-rates.html

[Tomcat088]

What everybody has said is spot on. The twist rate will depend on the weight of the bullet, and how long it. Bullets that aren't stabilized enough will never fly "true". It is the twisting of the bullet that gives it stabilization the same way that a gyroscope works. If the bullet is not stabilized by twist, it will tumble end over end (obviously not accurate). If they are overstabilized, the bullet may never go to "sleep" or settle into a twisting pattern around the bullets true center. Bullets that are over stabilized also don't nose over properly at the peak of the trajectory, I mentioned it in the other post about .243 twist rate. Here is a picture of what a bullet that has "keyholed" looks like.



Also something that happens when bullets are pushed very fast and they have too high of a twist rate, they sometimes just fragment and "explode" before they hit the target. You will usually see a slightly gray puff of smoke, or see fragments hit the ground and never make it to the target. This can also occur if there is a bur in the rifling of the barrel, and you are shooting thing jacketed bullets, or the jacket is badly damaged. Sometimes it just occurs from being spun too fast. Something else that is interesting is that even elevation and air density will affect the twist rate. Because of the different conditions, in less dense air, a bullet may not need quite as much twist to stabilize as it would in very dense air. There are times when shooters go to other states or locations with large differences and find out that their pet load won't shoot how it's supposed to. It's not usually a big deal, but if your bullets are right on the edge of stabilization for their weight, it does sometimes happen. It also sometimes happens that you barrel shouldn't be able to stabilize a bullet because it's too long or heavy, and the barrel might shoot that bullet better than the others. It's all quite interesting.

[VeryPrivate]

Greenhill's formula is the standard method for calculation the optimum twist for a given bullet.

Twist = 150 X D²/L
Where: D = bullet diameter in inches L= bullet length in inches
This formula had limitations, but worked well up to and in the vicinity of about 1,800 f.p.s. For higher velocities most ballistic experts suggest substituting 180 for 150 in the formula. The twist formulas used in the "Load From a Disk" program uses a modified Greenhill formula in which the "150" constant is replaced by a series of equations that allow corrections for muzzle velocity from 1,100 to 4,000 ft/s.

You can search Wikipedia under twist rate for more information.

[Pector55]

Picture Elway, Montana, Farvre, Namath, or Bradshaw thowing a football like any QB that every played for the Ravens. That's pretty much the difference between having the proper twist to spin the bullet, or simply chucking a piece of lead like it was a shot put.

[Montell C. Williams]

I think we are getting more info than Shilen Barrel Co. has.

[Qtrborecrazy]

Scallywag
Real easy, lets say a 1 in 10 twist, this means that for every 10 inches of barrel the bullet will rotate 1 time. If the rate of twist is 1 in 12 the bullet will rotate 1 time every 12 inches giong down the barrel. This increases or decreases the spin or rotation of the of a bullet. Some bullet weights need more spin in a certain caliber some need less. Does not sound like much but when the bullet is traveling at 3000 fps its spinning very fast.

Qtrborecrazy