What delegates twist rate

[wvredneck]

What delegates the twist rate for the long range bullets most of us use Charles E super moderator i expect you to chime in because i know you use Bibs flat base and i rather expect bearing surface is rather long.
Stan

 

[4Mesh]

WVrn,

Rate of twist is described in how many inches of barrel it takes for the bullet to turn one full turn. 11 twist means it turns once every 11 inches.

Basically, all bullets will be sold with a recommended twist rate specified and that number is the rate you need to spin the bullet in order to keep it going in a straight line.

You can overspin a bullet to a certain extent without ill effects (usually). Say, if you put a 190 30 cal out of a 9 twist barrel, that's a bit overkill, but, they've been known to shoot there too. More known for blowing up due to spinning so fast they come apart.

Usually, the length and shape of the bullet, and it's construction, determine what twist it needs. Take the 185 and 190 Berger VLD 30's for instance. They are the same bullet and measure the same length, or they did at one time. The difference in them is where the lead stops inside. The 185 has a bigger hole in the front. Those two bullets don't act exactly alike, tho, for the most part you could say they are close enough it doesn't matter.

I'm not sure if that's what you're looking for, but it's worth what ya paid.

 

[Joe Salt]

Wvredneck
Or do what I do, use a 1-10 twist and try them all. You may end up doing that just to find out what each barrel likes.
Joe SAlt

 

[R.G. Robinett]

What delegates the twist rate for the long range bullets most of us use Charles E super moderator i expect you to chime in because i know you use Bibs flat base and i rather expect bearing surface is rather long.
Stan

Counter-intuitively, when it comes to Sg, shank length (bearing surface) in near the back of the line. RG

 

[Charles E]

Dang dong (The chime of a southern Belle, which I ain't . . .)

Most of us use the calculations found on the JBL drag/twist site

http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi

An Sg of 1.4 is a good one, they aim for 1.5, which is certainly safe. You do need to know the measurement of the bullets, usually obtainable from the maker.

There are some disadvantages to using too fast a twist -- the groups won't be as small. As bullets aren't perfect, any difference between the center of mass of the bullet and it's physical center -- differences in jacket thickness, small voids in the core, etc. will cause greater dispersion the faster the bullet is spun. That's the compromise -- do you lean toward smaller groups, or stability in adverse conditions?

I've shot the 187 FB out of 13.5 twist barrels, Tooley used a 14-twist successfully. I don't like the cold; if it's under 50 degrees, I'm not shooting. YMMV. I'd recommend a 13-twist for the 187, which is made on an 1.300-length jacket. If you push them hard in a 10-twist, they'll blow up, like Joel did with his old .30 MHV barrel.

As far a manufacturer's recommendations go, I think Sierra a touch pessimistic. I've shot 240 SMKs out of a 10-twist. I believe, but do not know, that Berger uses the JBM site or something quite close. Witness Phil's loss of stability with a shorter barrel and the 12-twist using Berger 210 VLDs. Johnny Beyers used a 12-twist & Bergers successfully at Hawks Ridge -- 1,000 feet elevation & "not that cold."

Etc.

Edit:

Checking, link gave an error, so I fixed it.

 

[4Mesh]

WVRn,

Another example I mentioned in the other bullet thread is the Sierra 224 bullets. In a 14 twist, the 55Gn Spitzer will not fly. The 52Gn is actually a longer bullet, but it will. The 60Gn lead tipped bullet will also fly, and again, it is longer than the 55. Go figure...

Those sorta go along with what RR says above about length being less important than you might think. To look at the 55Gn Sierra alongside a 55Gn Nosler Ballistic Tip, you'd swear they were the same bullet all but the tip color. But, 'taint so. The Nosler 55 works perfectly in a 14 twist.

 

[mnakuchi]

Sir: Reference P233 chapter 10 paragraph 10.9 entitled Gyroscopic and dynamic stability of symmetric projectiles...This within the book entitled "Modern Exterior Ballistics" by Robert L. McCoy. Twist rate affects the stability of bullets as a function of bullet overall weight but shape too is very important. Try this thought experiment: Imagine throwing a standard NFL football as a spirial. Now, throw the same mass but elongated [simply a longer football having the exact same weight] this example approximates the stabilizing of a tanget ojive bullet [exampled by the standard NFL football-but now changed to a secant ojive shape that is longer but equal to the same mass]...Use your intuition to figure out which football is easier to throw as a stable rotating projectile. HINT: The shape of the elongated projectile challenges the thrower to generate a higher spin rate to stabalize the ball. The equations of motion shown in the afore mentioned reference are only correct within the limits of certain shapes/masses so again shape is an important characteristic [as is mass]. It is important to recognize that a rotating projectile can be over stabilized resulting in an increased drag component. As in a football toss ,the nose of the projectile must assume the attitude of the gravitational vector and hense point towards the target. [The roll over of a properly thrown football where the ball conforms to both aerodynamic drag and to the gravity vector results in the greatest distance and the best accuracy. Hope this helps----simply too much or too little stability are not good for long range accuracy....BE WELL! V/r k

 

[Charles E]

Essentially, a secant ogive offers more mass for the same length.

Another point is to consider a spear. It flies straight without any spin at all. Why? Because it flies slow, and the center of mass (that big honking tip) means the center of mass is in front of the center of pressure. Try throwing a stick with no tip added, where the big diameter is at the back. Oops. But that's what a bullet is.

Same with rockets having fins -- the fin moves the center of pressure way back.

That's why there are so many variables to fill in on the JBM site. It's a pretty good predictor -- Bill Davis's work. He was a friend of Walt Berger, and I'm pretty sure that something close to the JBM model is what Berger used. Berger's new ballistician, Brian Litz, may use a slightly more modern, more refined model, but the JBM drag/twist equations do a pretty good job.