Twist question

[Peter]

Why is it we use twists that "just" stabilize bullets of a given weight as opposed to using tighter twists?
Does it have more to do with bullets coming apart than accuracy?

 

[pbike]

Why is it we use twists that "just" stabilize bullets of a given weight as opposed to using tighter twists?
Does it have more to do with bullets coming apart than accuracy?

Robert Duval said it best in "Days of Thunder"... when he was explaining to Tom Cruise on how to set up the suspension on a Winston Cup Car. "Loose is Fast, but on the Edge of Out of Control"

With a .30 Br for score we want less Gun Torque in the bags so less twist, we want less friction in the barrel, so less twist, we want shorter lighter bullets, so less twist.

Paul

 

[John Kielly]

The better question might be, "Why do manufacturers generally only provide one twist per calibre when the shooter would be better off with one specific to his usage".

Target shooters use what they know will work the most elegantly - they pretty much control that environment; Joe Public can occasionally be perverse, anal, optomistic, but about always wrongheaded & manufacturers take that in mind.

 

[Wayne Shaw]

The faster you spin the bullet, the more pronounced any imperfections will be(balance, eccentricy). That is why BR bullets use jackets that are so near perfect in thickness, or as close to perfect, that can be made.

The bullet must be spun to stabilize it, but there is no point in spinning it more than necessary.

 

[John Kielly]

Damn, I just realised I didn't answer your question.

Generally speaking, the longer the projectile, the longer the bearing surface, the faster the twist that is required to stabilise it for accurate flight at any given velocity. Countering that, the faster you run any projectile, the less twist it will need to stabilise it. This is described in terms of a stability factor, a factor of 1 meaning stable, less than one unstable & more than one, doing fine. Our more conservative theorists say that a stability factor of 1.5 will belt & braces you for accurate shooting. The stability factor is based on a formula calculated from the shape of the profile of the projectile, more or less.

There are downsides to more twist, depending on what & at what distance you shoot. Too much twist can cause projectiles to strip or malfunction, going off in a puff of smoke before they reach the target. There's a reaction to twist, causing the rifle to torque & if that happens on a bench rest, it jumps the rifle off alignment. At long distances, the more spin given to a projectile because of twist, the more it will change elevation in variable winds, because the spinning cross section emulates an aerofoil's lift.

Factory rifles usually have belt & braces twists, except maybe Savage, who uses barrel twists that correlate to the usage they define the rifles for, so, for example you'll see a Savage Palma rifle with one twist, a F/TR of the same calibre with another. Target shooters who load to specific velocities select a twist that will j-u-s-t stabilise the projectile they're using at the velocity range that they will load it to. That way they get the positive reason for having rifling to start with & avoid the downsides.

 

[Charles E]

Questions that need an understanding of ballistics are usually not well answered on the internet forums. The topic is just too complex, with too many variables. A 10-page article, while not complete, would be about right to answer this question.

To give just a couple of details (NOT a thorough answer):

1. Bullets are not perfect. In the barrel, they are constrained to rotate around their physical center. In air, they rotate around their center of mass. Any difference between the physical center and the center of mass creates a force while the bullet is in the barrel, which is applied as the bullet clears the muzzle, so the shots disperse slightly, in a random manner. The greater the rate of twist, the greater the amount dispersion due to any CG-offset.

You can see evidence of this on paper at long range. Whether or not it is *significant* at short range depends on just how much wind there is, and how well other people shoot. Oh, and just how good the bullets are.

2. Gyroscopic stability varies with air density. Rate of twist then, needs to provide for variations in air temperature and altitude, even given fixed bullet characteristics and barrel twist rate.

3. The horizontal component of wind is greater with a higher Sg (gyroscopic stability).

4. Yaw, both primary and secondary, is affected by the rate of twist.

Etc.

All of these things involve compromises. Current opinion is that a Sg of around 1.4 is a very good compromise. You can probably live with about Sg 1.2 if, like me, you just won't shoot when it is cold.