what best twist for shooting 308 cast

[dombra]

does anyone have any opinions on the best overall twist?

[sundog]

If you are shooting cast, 12 is WAY better than 10. Depending on boolit length, 13 or 14 might be better.

[grouch]

Depends on the length of bullet you plan to shoot. 1 in 12 will handle bullets longer than the old 311284(220+gr in 20:1 alloy) and is moderately successful with 311365 which is longer.
I don't know where that leaves you with the new VLD jacketted bullets, but 1 in 12 certainly handles most cast bullets, and if you don't plan to load long bullets you could use a slower twist than that.

[joeb33050]

The Greenhill formula tells us the maximum length of a bullet that a given twist will stabilize. I use an approximation for translating bullet length to weight, (bullets weigh 82% as much as a cylinder of the same length), and here are those twists in ", max lengths in " and weights:

9", 1.58", 276 gr.
10", 1.42", 249 gr.
11", 1.29", 226 gr.
12", 1.19", 207 gr.
13", 1.09", 191 gr.
14", 1.02", 177 gr.
15", .95", 165 gr.
16", .89", 155 gr.
17", .84", 146 gr.

A fairly complete treatment of twist along with some EXCEL workbooks allowing you to fiddle with constants etc., are in the book.
joe b.

[cbrick]

Or if you would like to figure it out yourself for your different bullets,

Greenhill Formula:

The formula to determine the correct rate of rifling pitch for bullets of usual construction (lead alloy or jacketed) is known as the "Greenhill Formula".
Step One: Determine the length of the bullet in calibers. (Bullets length in inches divided by the bullets nominal caliber).
Step Two: Divide 150 (a constant) by the bullets length in calibers.
Step Three: Multiply the result of step two by the bullets "nominal" caliber. This will give the desired rifling twist rate.

Example: To find the correct twist for a .30 caliber 220 grain bullet 1.35 inches long.
1.35
150 = 4.5 calibers

150
4.5 = 33.33 x .30 = 9.999
or a 1 in 10 inch twist

Rick

[9.3X62AL]

Joe--that's a pretty handy "Greenhill Shorthand" for the 30 calibers. How variable is that .82 ratio of cylinder/boolit? Keeping in mind that I'm no math major, does this formula hold largely true for design ranges of, say, Lyman #311316 through #311413?

[Wayne Smith]

Deputy, the Greenhill formula was developed for artillery applications. I don't think it's that sensitive, although I've read somewhere of using a larger constant with rifle bullets because of it's original application. Seems to me that was on the old Shooters.com - anyone have the archives and can look up the Greenhill Formula on them?

[joeb33050]

Joe--that's a pretty handy "Greenhill Shorthand" for the 30 calibers. How variable is that .82 ratio of cylinder/boolit? Keeping in mind that I'm no math major, does this formula hold largely true for design ranges of, say, Lyman #311316 through #311413?
The first response got lost. Years ago I weighed and measured a lot of cast rifle bullets and came up with 82%. More recently I took the info from Don Eagans' catalog annd for the 21 bullet designs found ratios of 73.4% to 84.4% in Lino. The % will be greater for blunter fewer-grease-groove bullets, amaller for pointier or more-grease-groove bullets.
Maybe if we got samples of bullets of different shapes we could refine the approximation-"If the bullet looks like this, the ratio is 84%", but I donm't think it's worth it.
You can test it on the bullets you mention. See the book, Appendix, for the workbook "Alloy cylinder weights by caliber". Weigh your bullet, get WWW.W grains. Measure the length, get L.LLL inches. Get the weight of a 1" long cylinder of your alloy and the bullet's caliber, get AAA. grains, from the book.
Your bullet weight, WWW.W grains, divided by
Your bullet length, L.LLL inches, times the Alloy cylinder weight, AAA. grains
= the ratio, somewhere around 82% on average.
The book, up on a Yahoo site for free and for all, has workbooks for Greenhill and Charlie Dell formulas allowing much fiddling.
I don't know if that's clear or not. Tell me the weight and length and alloy of any bullet and I'll tell you the ratio.
Thanks;
joe b.

[sundog]

All that cyperin' is well and good. But you still have rpm limitations, low and high, unless you do other than ususal fit of boolit to bore, special design, lube, hardness, throating, etc.

For instance, how slow does the soupcan have to go in a 12 twist before it's unstable a 100 yards (not enough twist). How fast do you have to push the #301 or 311299 or 311284 to remmain stable at 200? It's not just length of boolit, it's also velocity (rpm) and max range (terminal ballistics).

I went to the range yesterday with the .308 and shot at 100. All loads were 34.0/4064 at ~2100 fps. Bbl is 12 twist. The L 311644 scattered all over the place. 311299 strung out horizontally almost in two different groups. The #301 shot 2-inch round groups. These three boolits are aproximately the same length and run 190 - 200 grains.

So, playing with the numbers might get you an approximation, but until you load it and shoot it, you won't know. I'll stick with my first opinion. The 12 twist is better than 10 in the .308. sundog

[Pat I.]

For an over the counter gun with cast bullets it'd be kind of hard to argue with what the .308 Savage varmint guns are doing and they have a 10 twist.

[9.3X62AL]

Wayne and Joe--many thanks! I had a basic "social science major-level" understanding of Greenhill, but that .82 percentage concept was new to me.

I get a little "spun up" about twist rates in pistol barrels, particularly 9mm and 40 S&W. Why a short, fat bullet used in those applications needs 1-10" or 4 turns/meter pitch eludes me.

[joeb33050]

So, playing with the numbers might get you an approximation, but until you load it and shoot it, you won't know. I'll stick with my first opinion. The 12 twist is better than 10 in the .308. sundog

I have no opinion about what twist is best in .30. If you're shooting cast 311299 at 1000 yards, maybe 10". Maybe 12" is better at 200 yards with cast. Also, look at the modern BR guys shooting shorter ~125 grain .30 bullets fast in 14", 16" and 17" twist barrels.
I think there's some evidence to suggest that the slowest twist yields the best accuracy, all other things being equal. To prove it would require much $$ and many barrels. But the modern BR slow twist guys may be onto something.

The gain twist guys had an instinctive horror at the thought of imparting an enormous rotational force on the bullet almost instantaneously; I can see H. M. Pope shuddering now.

For years we-the collective we- put 14"-16" twist barrels on 22 CF guns because we always did it that way. It took a few hours with a ballistic program and a BC estimator to find that to get a decent BC 22 bullet to shoot, we needed a <10" twist, down to 7". Savage is at 9" which stabilizes 80 grain bullets. It's only in the past few-20?-years that we figured out the 22 and twist.

Maybe we're at the same point on the .30.
joe brennan

[joeb33050]

All that cyperin' is well and good. But you still have rpm limitations, low and high, unless you do other than ususal fit of boolit to bore, special design, lube, hardness, throating, etc.

For instance, how slow does the soupcan have to go in a 12 twist before it's unstable a 100 yards (not enough twist). How fast do you have to push the #301 or 311299 or 311284 to remmain stable at 200? It's not just length of boolit, it's also velocity (rpm) and max range (terminal ballistics).

I went to the range yesterday with the .308 and shot at 100. All loads were 34.0/4064 at ~2100 fps. Bbl is 12 twist. The L 311644 scattered all over the place. 311299 strung out horizontally almost in two different groups. The #301 shot 2-inch round groups. These three boolits are aproximately the same length and run 190 - 200 grains.

So, playing with the numbers might get you an approximation, but until you load it and shoot it, you won't know. I'll stick with my first opinion. The 12 twist is better than 10 in the .308. sundog

I'm no ballistics expert but I am a very good reader, and everything I've read tells me that stability is affected very little by velocity The LRBP guys contend that a faster-than-Greenhill twist is necessary for their business; and have the data to prove it.
Your experience at 100 yards with 311299 at least is not caused by slow twist, I shoot many 311299/314299 at 100 yards at 1400 fps with accuracy ~ 1 1/4" bolt gun and <1" SS. 12" in the bolt gun, 10" in the SS.
joe b.

[joeb33050]

Wayne and Joe--many thanks! I had a basic "social science major-level" understanding of Greenhill, but that .82 percentage concept was new to me.

I get a little "spun up" about twist rates in pistol barrels, particularly 9mm and 40 S&W. Why a short, fat bullet used in those applications needs 1-10" or 4 turns/meter pitch eludes me.

I know little about revolvers and pistols. My 429421 is .77" long, weighs 248 grains in WW, is .432" diameter ~. It weighs 78% as much as a WW cylinder .77" long. (I'm surprised, I never did calcs with handgun bullets-thought they'd be higher than 82%) So, a .77" X .432" bullet has a minimum twist required of 36.4".
What is the twist in a 44? Is it close?
joe b.

[sundog]

Joe, how many shooters do you know that AFFECTIVELY and consistently shoot the 311299 at 1000 yards? To do this I'm sure a 10 twist or more and a VERY hard boolit is in order. Or an awful lot of trajectory. Special conditions.

I'm gonna stand on what I said. A 12 twist works pretty good, and until you load it and shoot it, you don't know.

Now, you not knowing the best twist for the .30, and you not being a ballistics expert but a good reader, and you who knows little about revolver and pistols, can have the last word. Whatever you say, I'm SURE you will be right.

[9.3X62AL]

Joe B--

Colt used 1-16" twist in their 44's, S&W uses 1-18.75", Ruger uses 1-20". All these twist rates seem to do well with "conventional" 44 caliber bullets (240-250 grain weights), and I haven't heard of problems with the heavy-for-caliber 300-325 grainers.