Does rate of twist = your bullet weight ?

Twist is going to determine what weight of bullet you can keep stable in given conditions.

A 1/10 twist in 308 will stabilize a 175 vld, but a slower 1/14 twist will not, and therefore you will have keyholes and poor accuracy.

For a .223 rem, I have a 1/9 twist. Usually I am going to run bullets lighter than 70 grains. YMMV of course.
For a .308 win, I have a 1/11 twist. I have had good experiences with bullets up to 190 grains. It would probably do others too.
For a .338 lapua, I have a 1/10 twist and am usually running a 300 smk.

Miller stability formula works well, google jbm ballistics. It will be on the bottom part of the page of calculators.

There are also some generic charts that you can google up I am sure.

Just seeing what other people are using successfully helps as well. Just remember that each rifle and conditions can be different.

Easy way to test is to shoot targets (maybe 12 ft, 25 yd 50 yd 100 yd etc) at various distances. Inspect the bullet holes; if they are nice and round you are good to go. Oval, then not so much. Check out the holes at whatever range you may be shooting just to be sure.

Easy way to find your twist is to mark a cleaning rod and run a patch down your bore. Measure how many inches it takes for the rod to make one full turn. If it takes 8 inches for one turn, then you have a 1 in 8 twist.

brow_tines said:

I was talking to another reloader the other day and he said he based his bullet weight on his barrel rate of twist. Therefore he shoots 62 grain bullets through his 22-250. Any thoughts on this?

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The twist rate of a barrel determines what length bullet the barrel will stabilize. Weight is not part of the equation. There are a couple different ones out there.

Conventional construction bullets, lead with a copper jacket, are pretty much the same length for the same weight. So a bullet weight can be associated with the appropriate length.

With non-lead bullets (all copper or non-lead cores), the bullets will be longer for the same weight as a lead bullet and therefore may need a faster twist.

The equations are not an exact science and several other factors come into play but bullet weight is not one of them.

brow_tines said:

OK, so is there a formula or chart to figure out rate of twist and bullet weights ?

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Twist = (CD^2)/L X Sqrt(SG/10.9)

The specific gravity of lead core bullets is about 10.9 so the second half goes away. For solid core copper bullets, substitute SG with the specific gravity of substance, 8.96.

Then twist for lead core bullets is (CD^2)/L. C = is a constant, use 150 for under 2800fps, if over 2800fps use 180. For about 2800fps, I use 165. D = diameter of the bullet and L = length of the bullet. Almost all of this info is from Wikipedia.

I use this information when I order a barrel from Satern. They can do rifling down to a thousandth of an inch. So I pick the bullet I intend to use the most and analyze it and get the twist rate this way. Then the barrel will be optimized for that one bullet, but will work fine with the others too. When you think about it, you can only have one optimized twist anyway, so if you have a barrel cut, get it cut to work best for what you plan on using.

The idea of "best" is a complete misnomer. "Adequate" is the correct term. If the twist is fast enough to stabilize the bullet, you're good. Beyond that, you can stress about it and develop paralysis through analysis to no practical effect. Decide the longest bullet you plan on shooting and plan your rate of twist to stabilize that bullet with a bit of margin for changing conditions and then get a high quality barrel. It will shoot that long bullet and anything shorter.

The twist rate of a barrel determines what length bullet the barrel will stabilize. Weight is not part of the equation.

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Cfull has it exactly correct. That's why one company's bullet will stabilize in a marginal twist barrel, when another company's bullet won't, even if they are both the same weight.

The idea of "best" is a complete misnomer. "Adequate" is the correct term. If the twist is fast enough to stabilize the bullet, you're good. Beyond that, you can stress about it and develop paralysis through analysis to no practical effect.

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+1. The Greenhill formula and any other formula used to determine the correct twist for a particular bullet, simply calculate the theoretical "optimal" twist rate for maximal stabilization. There is a bit of leeway in regards to stabilizing bullets.

Don

From Shilen

http://www.shilen.com/calibersAndTwists.html

This is general info - as stated above, the actual bearing surface of bullet to bore is the key, but bullet weight will get you in the neighborhood.

Barrel Rate of Twist

In .224 bullets, Sierra 69gr & Hornady 68gr Match have the required twist printed on the box. Sites like Berger has twist rates posted for each bullet. Sierra only for some that require a faster than normal twist. The velocity of a bullet in a given caliber has to have an effect? Compare 222 Rem. to 220 swift? Talking about length, the Hornady 68 is a lot longer then the Sierra 69gr match, takin up way to much powder space in the 223/5.56.

Yes with a higher velocity you can get away with a lower twist rate. After all the bullet doesn't know what rate of twist you are using. It's the RPM's that matter.

Even after finding the bullet weight that's supposedly good, you have to remember each rifle is different.

In a .223, for instance, mine is a 1 in 9 twist.
I've tried 50, 55, 60 & 62 grains.

The 55 grain is far superior to the others in accuracy.

In this case everyone's mileage WILL vary.

"Adequate" is the correct term. If the twist is fast enough to stabilize the bullet, you're good.

Well stated.

It's impossible to do much figgering on twist rate alone, it's spin that determines stability and spin is a function of both twist and velocity.

if the bullet shoots well in your rifle, shoot it. if it doesn't, don't shoot it.

murf

Velocity plays a part because pushing the bullet faster does increase the RPM of the bullet, but it also increases the upset force on the bullet requiring it to spin faster to remain stable.

Cemetery21 said:

http://www.shilen.com/calibersAndTwists.html

This is general info - as stated above, the actual bearing surface of bullet to bore is the key, but bullet weight will get you in the neighborhood.

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I looked up my new .270 wsm.... Says 10" for ALL bullets...

My Remington XCR 700 says it has a 10" twist... Sa-Weet!!!

Over Stabilization

IIRC, Julian Hatcher stated in a book authored by him titled "Hatcher's Notebook" that a bullet can be over stabilized. If memory serves, the problem occurs when the over stabilized bullet reaches the apex of it's trajectory and resists going over point first for a period of time.

I have the book, but it's currently in one of many unopened boxes resulting from the move to my new digs. I'll try and get some specific references unless someone knowledgeable can jump in here.....Doc