Someone explain barrel rates of twist

[Rob]

OK, while I have been hunting for several decades, and I know what a twist rate is (1 in 12 = one bullet revolution in 12 inches, smaller 2nd # = faster rate of twist, etc...), can someone explain in more detail (or point me to a thread or website) rules of thumb for twist rates?  for example:

-What are the ranges (I see numbers around 1 in 12 often enough but I assume 1 in 300 would never happen, nor 1 in 3, etc)
-What does a faster twist rate provide?  (I assume more stable flight?)
-Are there rules of thumb for different calibers (i.e 1 in 12 is great for a small caliber light bullet, but would suck for some other caliber...)
-Are there hunting applications where a faster twist rate is better?
-Does the twist rate impact velocity or energy?  If so, how?

Anything else?

[SpokaneSlayer]

http://www.nfa.ca/content/view/129/197/

I found this. 

[Rob]

Cool table.

In looking at it, in general as a rule of thumb, within a caliber, the heavier bullet you shoot, the faster you want the twist rate.  And the smaller caliber you shoot, the faster you want the twist (i.e. with one exception the 1:8's drop off once you cross the 30 cal)?

Or is that too simplistic?

[whacker1]

I wonder how barrel length would affect the table.  It would seem another 2 inches of barrel would have the additional rotation and possibly require less twist?  All speculation on my part as this is a question that I have always had. 
recently thinking about it for my muzzleloader.

[Bofire]

 :)This question cannot be aswered simply. Heavy bullet fast twist or light bullet slow twist. How fast is this cartridge to start with? 2000 fps or 4000fps? What type of bullet construction? some come apart in a fast twist. A high velocity might improve from a higher twist but you might also get more heat, more pressure and less barrrel life.
At long range a 223 that starts out slower than a 22-250, but has a faster twist, with a heavy bullet for caliber, will out perform the 22-250 accuracy wise. (assuming equal quality rifles)a fast twist 22-250 is super but heats up, has high pressure and short barrel life. and I mean much shorter life, like maybe 300-600 rounds.
Like most physics it is simple but very complicated.
Carl

[jjhunter]

Length of barrel alone has no effect on the twist rate's effect on the bullet.  When I build a rifle, I look at two things 1)  Bullet that I will be shooting  2) Speed I will be shooting the bullet at.  With a hyper-velocity catridge, you can get away with a slower twist barrel in some cases as the speed at which the bullet is pushed makes up for the slower twist barrel.  It is important to discuss these items with your smith, and also, talk to bullet maker.  They can both be a wealth of knowledge and save you money in the long run.   

[WDFW-SUX]

bingo.

[whacker1]

Interesting - I would have thought that the hyper velocity cartridges with a slower rate of twist in a longer barrel would give it additional opportunity to stabilize.

Thank you folks for the physics lesson....

[Rob]

what is the definition of a hyper velocity bullet?

[AWS]

Twist rate relate to bullet length not weight.  With lead core bullets weight and length are pretty constant so you can recomend twist to weight , when you add in solid copper and lead free cores weight goes out the window and length of the bullet is the basis for twist rate selection.   223 cal bullets, 36grain Varmint Grenades(tin core)  55gr Lead Core and 70gr Dead Coyote (tungsten core) all take the same ideal twist rate 1-12 to 1-14.  One thing you can always over staballize a bullet and still get great accuracy ie. 40gr bullets will shoot great in twist from 1-8 to 1-16.

The actual formula for finding twist rates is called the Greenhill Formula.

http://www.geocities.com/capitolHill/lobby/1221/grn_hill.htm


AWS