Twist rates for cast bullets as compared to jacketd bullets ?

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Last Post 03 December 2018

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Slug Gun posted this 24 November 2018

I’m just now getting into casting bullets in .30 cal. and want to build a rifle and chamber it in 30BR. I see from the FS equipment lists that 200 thru 230 gr. bullets seem to be popular for that cartridge and most shooters use a 12” twist for those bullets. Now that I want to order a barrel, from jacketed bullet experience, a 10” twist would be needed but I’ve noticed that the overwhelming majority of competitors are using a 12” twist.

Is there a reason that lead bullets require a slower twist as compared to jacketed bullets ? A 12” twist wouldn’t come close to stabilizing jacketed bullets of that weight but a 12” twist seems to work best with lead bullets. Why ?

THANKS

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joeb33050 posted this 24 November 2018

30 caliber, .308"

10" twist, bullet max length 1.423", ~ weight 250 gr

12" twist, bullet max length 1.186", ~ weight 207 gr

14" twist, bullet max length 1.016", ~ weight 178 gr

16" twist, bullet max length .89", ~ weight 156 gr

All per Greenhill.

There's lotsa agreement that slower twist = more accurate, agreement, but little data.

Velocity assumption ~1400 fps, BUT, velocity has little effect on stability.

When the BS concludes, a 10" twist is your best choice.

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frnkeore posted this 24 November 2018

The Greenhill formula is obsolete and will give a result with to fast a twist, except in LR, cast bullets (600+ yds).

The reason is that the GH formula, doesn't solve for center of gravity.

For the last 30 years, I've shot 1.16 - 1.19 long bullets, in my 14 twist Douglas barrel. 1450 fps range.

I have a new gain twist RKS ending in 13 that stabilizes 1.27 long bullet. 1450 fps range.

I have a A&M 12 twist barrel that I've shot 5/16" groups, with 190 gr HPBT and 200 gr Speer Jacketed bullets. 2000 fps range.

One of the most common match bullets for CBA competition, is the MX4-30 ARD, it's shot in 11 and 12 twist rifles and is 1.25" long. You can shoot bullets at least 1.30" long in a 12 twist barrel at 1450 fps and I think you can shoot at least a 1.33" long in the 2000 fps range.

Frank

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Bud Hyett posted this 24 November 2018

The Greenhill Formula is empirical, derived from experimental data and observation. This derivation does not invalidate the formula, rather the formula is a tribute to the meticulous work done in the previous century to establish an usable formula.

There can be another factor in the discussion of stability, the access to better barrels across the mainstream. The custom manufacturers today produce better barrels in the main than the old timers. A smooth journey down the bore can add to the stability in flight. Exiting the muzzle with a crown and the rest of the muzzle perpendicular to line of flight, aids a bullet to be stable.

The Greenhill Formula is a guide, not an absolute.

Farm boy from Illinois, living in the magical Pacific Northwest

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Ross Smith posted this 24 November 2018

Don't let lead bullet weight throw you a curve. Cast bullets weigh more per unit of length than jacketed bullets. So it is more a function of length and rpm's. I'm shooting 1.15" cal bullets in a 14" twist hart barrel, but I think the 12" would be more universal especially at lower velocities. I just got my 30-06 back with it's 1-14" barrel and will see how it shoots as soon as our storm moves out. It has been a quandry for me too needing slower twists in cb barrels but I think a light short 308 j-bullet would shoot well in a slow twist barrel also. have fun and good luck.

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Hornet posted this 25 November 2018

Greenhill is NOT obsolete, is not empirically derived, the 150 constant is NOT wrong. What you have is people taking the SIMPLIFIED Greenhill (which is kind of an Executive Summary/For Dummies version) and trying to apply it to everything without understanding what the constant represents. That 150 was the result of solving the full mathematical development as applied to the .30-40 Krag/.303 British cartridge (about identical ballistics) -220 Grain RN .30 cal 1.35" long at 2000 fps with a density of 10.9 gm/ml, etc. The further you change things from this, the more you need to change to a different constant.So, no the 150 doesn't give the best twist for your 350 grain semi-wadcutter from a .50 muzzleloader OR for a 120 grain boattail VLD HP from your .240 Weatherby.

You need to know what goes into the constant and how a specific characteristic is applied in order to rationally change it. See the article on here https://www.fulton-armory.com/faqs/AR-FAQs/ARTwists.html for a full discussion. Take an aspirin first, it's tricky.

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frnkeore posted this 25 November 2018

I call it obsolete, because there are at least 2 others and one that gives accurate solutions to any given set of circumstances. The one that I use, solves for the bullets center of gravity, as well as weight and length.

Why would you want to reverse engineer for your particular situation? Back in the early 90's, I did that with the GH formula and found that a constant of 175 worked well with spitzer cast bullets, in the 1400 fps range but, the program that I now use, will solve for any type of bullet at any velocity, including subsonic and secant nose profiles. It's the only one that I know of that will.

Frank

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joeb33050 posted this 25 November 2018

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joeb33050 posted this 25 November 2018

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joeb33050 posted this 25 November 2018

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joeb33050 posted this 25 November 2018

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joeb33050 posted this 25 November 2018

Greenhill never fails.

I have available to any and all an Excel workbook that allows one to calculate twist numbers for 4 different twist formulas, including Greenhill. You have but to ask.

Greenhill never fails, and is the only formula with a SINGLE solution.

Greenhill never fails to bring forth an abundance of opinion little of which is from those who have read Greenhill's work.

A 10" twist is my recommendation.

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Slug Gun posted this 25 November 2018

I seem to have stirred-up a hornets nest here. Previously, I went to Castpics.net then Reloaders Reference then Twist Rate Calculation and put in some numbers. That site says Modified Greenhill Formula and I took that to mean that it “probably” uses the 150 constant. Here are the twist rates from that formula for a .308” bullet:

1.125” length at 1800 FPS twist= 12.52”

1.125” length at 2000 FPS twist= 13.20”

1.182” length at 1800 FPS twist= 11.92”

1.182” length at 2000 FPS twist= 12.56

I’m guessing these are the “minimum” rates so to be on the safe side I am thinking that a 10” twist would be best to be on the safe side of being stabilized.

Now, back to my original question:

is there a reason that lead bullets require a slower twist as compared to jacketed bullets ? A 12” twist wouldn’t come close to stabilizing jacketed bullets of that weight but a 12” twist seems to work best (from reviewing the CBA equipment lists for the 30BR) with LEAD bullets.

Is there a problem with lead bullets not staying engaged with the riflings of fast twist barrels ? Can lead bullets not stand the higher RPM’s of fast twist rates ? I’m new to this game and trying to understand this issue.

THANKS for the info and posts.

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joeb33050 posted this 25 November 2018

Now, back to my original question:

is there a reason that lead bullets require a slower twist as compared to jacketed bullets ?

In practical terms, NO. In scientificy terms, yes, because min twist varies with bullet density, denser / slower. And, because cast have apparent density > jacketed.

Min twist varies with bullet diameter, length and density, velocity, and air density.

30 cal 10" twist barrels will never let you down. Slow or fast, short or long, lead or lino, sea level or mountains.

Is there a problem with lead bullets not staying engaged with the riflings of fast twist barrels ? Can lead bullets not stand the higher RPM’s of fast twist rates ? I’m new to this game and trying to understand this issue.

No.

THANKS for the info and posts.

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Eutectic posted this 25 November 2018

Slug,

The problem of cast bullets "jumping the rifling" was addressed by E.H.Harrison in the NRA publication "Cast Bullets" page 14.

Unlike rotational stability, the math is simple. In summary, even in a "worst case" 30 caliber, 4 groove barrel, ten inch twist, 30,000 PSI load, short body bullet with a long nose. Even moderately hard ( BHN 16) alloy is easily capable of providing the necessary torque to spin the bullet.

This assumes the bullet is in the bore at the peak pressure. Running the math with the bullet hitting the rifling at speed, which happens in revolvers and in a rifle with long free bore, is quite different. The instantaneous torque is much higher and stripping can occur.

Steve

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RicinYakima posted this 25 November 2018

I agree with Steve and joeb. Some of the reputation of fast is bad is from military rifle chambers from the 1890 -1920 period. The chamber is cut for long straight sided bullets. Some like the 7.65 Argie and 6.5 Swede have over an inch of groove diameter throat so any type of bore riding cast bullet is at a distinct disadvantage. The common solution is softer alloy and slower powder. Bullet obturates up to groove diameter and peak pressure doesn't happen until the bullet is fully engraved.

Without a chamber cast, you are just spitting into the wind. Bullets do not straighten out once the case expands; if it is at an angle when it enters of rifled bore, it will leave the rifled bore at the same angle.

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Slug Gun posted this 25 November 2018

Steve

I checked my stash of old books and sure enough, I have the 1979 NRA book, “Cast Bullets” and read page 14. I forgot about this book that I bought back when I was into revolver bullet casting. Looking over this book, I think I’ll read it again because it covers all the subjects that I currently have so many questions about. Thanks for the “heads-up !

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John Alexander posted this 25 November 2018

Slug Gun,

I shoot a 223 in competition and in order to shoot a long enough bullet to be competitive at 200 yards (longer length higher SD and thus BC). An 8" twist is best for my bullets, 9" minimum. I don't feel at a disadvantage shooting against 30 caliber shooters shooting their long bullets in slower twists. So I have some opinions on fast twists.

There are two possible disadvantages with a twist sharper than you need.

1. The effect on accuracy of any defect will be greater the faster the twist (especially if the defect is near the surface). This theory is based on basic physics and pretty hard to doubt if open minded. However, with decently cast bullets the magnitude of the actual effect is apparently pretty small to the point of insignificance or I would be in big trouble with an 8" twist and my unsorted and unweighed bullets, but I don't see that happening.

Sorting out the visible defects and sorting by weigh would reduce this effect but the improvement is apparently so tiny that I can't prove it by testing no matter how many times I try. So, although the effect is real, with reasonable bullets it is apparently too small to show up at the target. Learning to cast consistently will yield more results than going with a twist that will barely stabilize your bullets.

2. The other possible problem if that the faster twists will make it more likely that the higher force the lands put on the bullet to spin it will widen the land marks in the bullet enough to let hot gasses through and cause trouble. Recovered bullets do sometimes show skids but whether that causes problems is unproven.

Some people also say that fast twists "over stabilize" the bullet but I have no idea what the hell that means. Can't imagine anything being too stable.

John

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Ed Harris posted this 25 November 2018

Great discussion here.

But cutting to the chase, with quality bullets and barrels, a bit more twist than needed hurts less than having not quite enough.

Greenhill works well in our cast bullet situations, with blunt bullets at lower velocities less than 2000 fps.

With subsonic loads a faster twist helps.

With the highest velocity which is usually practical with cast bullets, over 2000 fps, then a slower twist helps.

But, if the barrel is good, the bullets are good and your load is well-balanced for the alloy and caliber, much of our twist discussions are mental masturbation.

My most accurate cast bullet rifle is an Obermeyer 5R with 13.7" twist in 7.62x39 which I shoot 190-grain cast in about 1800 fps.

My 14" twist .30-'06 is also wonderful with #311299 and heavy charges over 2000 fps.

But I also have a 7.8" twist .30-'06 which stabilizes 240-grain bullets at the lowest velocity which exits the barrel and is little louder than a .22 LR firing standard velocity match ammo.

My favorite woodchuck rifle is a 7" twist .22-250 which is 1/2 moa with Sierra 53-grain HP benchrest bullets and 36 grs. of 4064. Bullets leave a blue smoke trace and do not exit a 200-yard woodchuck, but inflate them like a ballon, leaving 2 quarts of strawberry jam in a leather bag.

Joe and Ric are right. Take care of your good barrel.

Be fussy in visual inspection of your bullets. Weighing matters only to discard obvious outliers.

Have fun. Don't over-think the problem.

73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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Slug Gun posted this 25 November 2018

John,

I can make a comment on over stabilizing bullets but only with jacketed bullets. I’ve seen it happen twice, both times in .223. The first time was bullets blowing up about 20 feet in front of the muzzle of an AR-15 with a 7” twist barrel and the shooter was shooting 45 gr. Varmint bullets. The RPM and thin jackets caused the bullets to explode. The second time was in F-class and a shooter was shooting 90 gr. bullets with the manufacturer recommended 6 1/2” twist and he had a 30” long barrel. Since the twist was the recommended rate, he disregarded that and blamed the barrel. The manufacturer sent him a new barrel. He rebarreled with the same 6 1/2” twist 30” long and that barrel did exactly the same thing, the bullets could be seen to explode about 50 to 75 feet from the muzzle. He rebarreled again but this time to a 7” twist but only 28” long and never had a problem after that. Same 90 gr. bullets. I saw both rifles do this so thats exactly what was used and what happened. Maybe these two scenarios could be blamed on “over stabilizing” ???

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Ed Harris posted this 25 November 2018

Jacketed bullet blow ups are generally caused by engraving of sharp-edged rifling producing stress risers in the jacket material, which if the jacket material is of inadequate strength, given excessive rotational velocity, hoop stress can exceed the shear strength of the jacket material.

73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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