Ideal twist rates for cast boolits vs. jacketed. Different? Why?

[MBTcustom]

This is a pretty deep question that involves so many variables its almost impossibe to answer, but im going to ask it anyway because im trying to get my head around it.
I’m not trying to start a firestorm here, but I would like to get a better handle on rifling twist rates as they pertain to cast lead boolits.
There doesn’t seem to be any tried and true measure of bullet performance vs. twist rate except the Greenhill formula, which seems to be little more than a ballpark guesstimate.

I have called Kreiger and asked where to go to find good information on this subject, and I was told that each projectile is a law unto itself and when ordering a barrel, to call the bullet manufacturer and describe the length and contour of the barrel, and speed of the shot and follow their recommendations for twist rate. But he also said that those numbers can be fudged one way or the other by powder selection etc etc etc. (surprise surprise)

The water gets even muddier when you bring cast boolits into the mix. Each individual caster is the manufacturer in this case, and it seems that the sweet spot of the boolits is dependant on each caster’s alloy, quality and boolit design, which could dramatically affect the ideal rate of twist.

I know that this could be a question that has no answer, but it would be very good if we could hammer out a few basic rules of thumb, that could be used to help make the jump from A to C.

Just in 30 caliber, I have personally witnessed excellent groups being shot at 100 yards (excellent being under 1.5”) with barrels twisted from 8 to 14 (albeit at many different speeds and bullet styles/designs)

The basic rule of thumb that I generally accept as being true are is that if you have a longer projectile/diameter ratio, you need a faster twist.

This rule works for boolits and jacketed, but each style projectile functions in its own window of effectiveness, and when you make the change from jacketed to cast, you have to rewrite the book.
Or perhaps there are problems with the cast lead boolit that mask the issue and make it seem like you have to rewrite the book when in fact the projectiles are following the same patterns regardless of their hardness.

For instance, say we have a 30-06 with a 12 twist barrel, and we know that it likes 175 Sierra Match Kings driven to 2750 FPS. That’s where the groups go to sleep and start bugholing with that particular load. Groups measure .479 MOA.
So then we decide to go to a 220 grain cast lead boolit of similar shape, and find that it works best at about 2000fps, and gives us a group of about 1.5
Now, say we take a 220 grain jacketed bullet and run it right at 2000 FPS. Will we find that it has trouble stabilizing? Will it only give groups of about 1.5” (kinda like the cast lead boolit?) till we start ramping up the speed a little bit?
Furthermore, was the cast lead really working all that well at 2000, or was that just the fastest we could push that particular boolit, cast by that particular booliteer, with that particular alloy before accuracy fell apart due to inconsistencies?

Now, before anybody jumps all over me about the inaccuracy of the above example, please understand that it’s not real, and I dreamed up the whole scenario to try to get the point across and describe something that is very hard to put into words.

I guess what I’m asking is if we are being biased towards cast lead because it has trouble at high RPM? Or do the rules change dramatically when we make the switch? If so, Why?
Are there “RPM nodes” that a certain projectile will settle into, or does rate of twist really have much less to do with the stability of the projectile than barrel harmonics do?

I don’t have the answers, and it’s such a complex issue, it’s hard to even put it all into words.
I would appreciate a civil discussion about this. Feel free to put your two cents in, but please resist the urge to make someone else see things from your point of view.
There is no hard science on this subject, especially for cast lead, so I’m interested in any point of view on the subject.
What am I not considering (take your pick from the 55 gallon drum of variables)?
What’s your opinion? Why do you see it that way?
Thanks!

[geargnasher]

I read that twice and still haven't a clue what you're talking about. There are quite a few good books dedicated to the subject of exterior ballistics, if that's what you're asking.

Gear

[MBTcustom]

There's people getting good results with cast lead at half the velocity of jacketed. WHY?
If I get off on the twist rate just a little with jacketed, it can take a bughole to a 1" group instantly, but not a lead boolit. WHY?

Cast lead seems much more tolerant to twist than jacketed is. Is that the truth, or do we hold cast to a looser standard? Is it just perception?

Oh and I'm thinking more along the lines of interior ballistics I think. Actually, it's kind of the whole enchilada, Exterior and interior and why they work the way they do.

Some say that cast is a whole new ballgame. Some say there is no difference, except that the rules change for slower projectiles no matter what they are made of.
In other words, some people say that a 30-06 in 10 twist will shoot jacketed or cast equally well if you know what you are doing, but very few can get a cast lead boolit to go faster than 2000fps with that twist.
It seems that the ideal projectile/speed changes dramatically when we go to cast.....or does it?

Just thinking out loud.

Of course, if you would rather tell me how to get the wrinkles out of my boolits or why my barrel is leaded, we could talk about that instead?

[243winxb]

High RPM's with cast = Skidding in fast twist barrels made for Jacketed bullets. Easy to see if using this calculator?? http://www.bergerbullets.com/litz/TwistRuleAltWP.php A Lyman 308 dia. 171.5 gr gas checked bullet at 1502 fps to 2000 fps will work ok with a 17 twist barrel. Bullet length of .968" A Hornady jacketed RN 150 gr with a 18 twist is OK from 1960 fps to 2700 fps. Bullet Length of .909" The Sierra jacketed 150 gr Spitzer at 1960 fps needs a 14 twist. At 2700 fps also needs a 14 twist. Bullet length is 1.068" It would be nice if it was this simple. But its not. In the real world, without calculators, testing is the only way to know. Interesting post. Cant wait to see how wrong the above info is.

[MBTcustom]

High RPM's with cast = Skidding in fast twist barrels made for Jacketed bullets. Easy to see if using this calculator?? http://www.bergerbullets.com/litz/TwistRuleAltWP.php A Lyman 308 dia. 171.5 gr gas checked bullet at 1502 fps to 2000 fps will work ok with a 17 twist barrel. Bullet length of .968" A Hornady jacketed RN 150 gr with a 18 twist is OK from 1960 fps to 2700 fps. Bullet Length of .909" The Sierra jacketed 150 gr Spitzer at 1960 fps needs a 8 twist. At 2700 fps a 9 or 8 twist. Bullet length is 1.606" It would be nice if it was this simple. But its not. In the real world, without calculators, testing is the only way to know. Interesting post. Cant wait to see how wrong the above info is.

This is exactly what I mean. Go to jacketed, and suddenly your stabilizing SHORTER projectiles at higher speeds.

[MBTcustom]

OK, so if you take a 175 SMK going 2800fps, according to the calculator, a twist rate of 15.6 will barely stabilize that bullet with a SG value of 1.01
Lets just call it a 16 twist.
Now, drop that back to cast boolit speeds and see what happens.
2400 FPS gives a SG value of .96 which they are calling horribly unstable. Yet some booliteers claim that's exactly where they want to run their cast lead boolits.

So now, punch it in at a standard twist of 10 and bump the fps back up to 2800. You get a SG value of 2.45 which is exceedingly stable. Like heck and gone stable already!
But a cast lead boolit won't run there for whatever reason. We bump the FPS back to the standard > 2000 FPS which gives us a SG of 2.19 which is still hanging right in there in the (comfortable stability) area.

It seems that we would want the fastest twist that we could possibly get away with. Even with cast lead.

What started all this was a conversation that I had with Bjornb about barrel twist. He mentioned that he was looking at getting a 16 twist in a 30 XCB and I was of the opinion that I wouldn't go any slower than 14. The only thing we are fighting is boolit damage at engraving, other than that, I would think that more stability is better.
It's a trade off between external ballistics and internal ballistics. External ballistics wants MORE MORE MORE!!! While internal ballistics wants LESS LESS LESS!!!!

Is there anything else at play here?
Seems the real trick is to get that boolit engraved without damage (or whatever happens in there) and if you can do that, then the faster you can spin the better. More is always better when stabilizing a boolit in flight.
Or is it?
Is there some other factor that is attached to soft lead boolits that jacketed does not have to contend with?
Look at your calculators. They all say the same thing.

[243winxb]

Jacketed bullets go to sleep or stabilize at some point in there flight. Before they stabilize, the nose is running high & left from a right hand twist barrel. The bullet is pushed off course more if the wind hits it before stabilization. Excess RPM's are not your friend. Plus the torque to the whole rifle is more with a faster twist.

[Fluxed]

snipped a whole bunch....
Cant wait to see how wrong the above info is.

I'm generally agreeing with what you wrote but this part must be a typo:
"The Sierra jacketed 150 gr Spitzer at 1960 fps needs a 8 twist. At 2700 fps a 9 or 8 twist. "

[MBTcustom]

Jacketed bullets go to sleep or stabilize at some point in there flight. Before they stabilize, the nose is running high & left from a right hand twist barrel. The bullet is pushed off course more if the wind hits it before stabilization. Excess RPM's are not your friend. Plus the torque to the whole rifle is more with a faster twist.

I'm willing, but why is there no red line for RPM's in the calculator? How blazin fast do you have to be turning in order to have "excess RPM's"? Why does a cast boolit need less RPM's than a jacketed bullet (assuming you get it out of the barrel undamaged)?
I built a rifle for Love life that was a 243MG with a 7.5 twist barrel. The bullet literally made three complete revolutions before it exited the barrel. The bullet was spinning right around 300,000RPM, and it was almost 1/2MOA out to 1000 yards. If ever a bullet was over stabilized, that would be the one eh?
Also, I was under the impression that a bullet would "go to sleep" very soon after exiting the barrel (like this side of 50 yards).

I think that twist can be overdone or underdone, but for a 30 caliber, I wonder if the 10 twist isn't just altogether unnecessarily fast for both cast and jacketed. either that, or it's just about right for jacketed and cast, but we've got to figure out how to get the boolit out of the barrel undamaged in order to realize the "catch all" benefits of the faster twist.

[str8shot426]

I'm willing, but why is there no red line for RPM's in the calculator? How blazin fast do you have to be turning in order to have "excess RPM's"? Why does a cast boolit need less RPM's than a jacketed bullet (assuming you get it out of the barrel undamaged)?
I built a rifle for Love life that was a 243MG with a 7.5 twist barrel. The bullet literally made three complete revolutions before it exited the barrel. The bullet was spinning right around 300,000RPM, and it was almost 1/2MOA out to 1000 yards. If ever a bullet was over stabilized, that would be the one eh?
Also, I was under the impression that a bullet would "go to sleep" very soon after exiting the barrel (like this side of 50 yards).

I think that twist can be overdone or underdone, but for a 30 caliber, I wonder if the 10 twist isn't just altogether unnecessarily fast for both cast and jacketed. either that, or it's just about right for jacketed and cast, but we've got to figure out how to get the boolit out of the barrel undamaged in order to realize the "catch all" benefits of the faster twist.

Twist can be underdone and overdone. Overdone meaning, more than needed to stabilize the projectile. But I do not believe a projectile can be over stabilized.

[243winxb]

Good catch. Bullet length entered was wrong. Its 1.068" I though that didnt look right. A 14 twist for both velocities

I'm generally agreeing with what you wrote but this part must be a typo:
"The Sierra jacketed 150 gr Spitzer at 1960 fps needs a 8 twist. At 2700 fps a 9 or 8 twist. "

[RPRNY]

I want to be respectful but you're sending out those vibes that say "I've got a bee in my bonnet and while I've said I'm looking for information, I'm really looking for affirmation".

Greenhill is your rule of thumb. There's a Greenhill Corollary for Boolits that says something to the effect of,"Generally speaking, cast boolits the same weight as jacketed will be slightly shorter, will stabilize at a slightly lower twist rate and will do better at lower velocity." Again, that's a rule of thumb, not a commandment.

Same weight, similar bearing surface, decent lube, a cast boolit of the same weight as a jacketed one will be somewhat faster and somewhat lower pressure. Skidding and smearing were discussed above and high velocity/ fast twist will often get you there. Another rule of thumb: heavy for caliber boolits seated close to or just off the rifling and pushed at a more sedate pace than the same bullet weight wearing a jacket will almost always do better than a smaller, faster boolit in the same caliber and rifle.

So, if you're building a rifle and want to shoot long for caliber jbullets from it, pick the fast twist and when you want shoot boolits with it, go big and slow. If cast is your game, back off a little on twist and shoot shorter jbullets. The bigger question is in throat design...

Now several people are going to come along and tell us how well their pet 85 grs boolit hard cast, under bore, and lubed with PAM does in their 257 Weatherby Magnum at 3100 fps and all these general rules of thumb are rubbish. That's fine.

[MBTcustom]

Twist can be underdone and overdone. Overdone meaning, more than needed to stabilize the projectile. But I do not believe a projectile can be over stabilized.

Right. Obviously, the 30-06 in a 10 twist is over stabilized by a pretty good margin.

[beagle]

Goodsteel......you done opened a can of worms here. Cast and jacketed are two different cases as we're all aware of.

Jacketed bullets are more uniform internally than cast and can withstand more velocity. We take that as a given.

With the Greenhill formulas floating around (and there are many), there are three variables that we can do anything about: bullet diameter, bullet length and velocity. A lot of sins can be overcome with velocity but here with cast bullets, we encounter skidding when we go too fast.

One winter, Felix and I messed around all winter shipping bullets back and forth between Arkansas and Kentucky and playing with twist rates, velocities and different length bullets in the .44 Magnum.

I had a Marlin M1894 1:38 and it wouldn't shoot cast worth sour grapes.....especially 300+ grain bullets. I sent it off and had a Douglas 1-20" twist barrel installed. Now, it shot 275-320 grain bullets but not lighter. Finally, I bought a Winchester Legacy M94 with a 1:26 twist and it shot the 240 grain Keiths just fine and the big bullets with acceptable accuracy so I quit while I was ahead.

From this experiment and our models with all bullets tried, we concluded that the ideal twist for a .44 Mag was 1-25 and a fraction based on the Greenhill version we were using and our range results showed us that we were on line.

The 1-20 twist was definitely over stabilizing the lighter bullets and this is the question I think we're talking about.

At this point in time one of the old members (Petey)was experimenting with impact coating cast bullets with moly and using no lube. I was looking at his rejects and he was getting voids in about 75% of his bullets. These were internal voids that weren't visible until after the tumbling operation for the moly coating.

Felix and I figured that this was the cause of inaccuracy in over stabilized bullets.

Yes, if the bullets were perfect, accuracy would be there even if over stabilized. If not, the over stabilization caused the void to unbalance the bullet and cause wild gyrations at over stabilization and inaccuracy on the terminal end.

I was really amazed at the number of voids in his and some .30 bullets I cast for him to coat that I thought were perfect. I mean on the ogive, base, in the bands and on the front driving band as well. What we thing are perfect bullets....aren't./beagle

[bnelson06]

My thought is that this is going to get interesting and I will be following closely

[MBTcustom]

I want to be respectful but you're sending out those vibes that say "I've got a bee in my bonnet and while I've said I'm looking for information, I'm really looking for affirmation".

Not particularly, and I take no offence to candid opinion. I apologize if I came across that way.
The thing is, I know how to recommend a twist rate for jacketed bullets, and I build rifles for that purpose. It's all very easy and well documented. 30 caliber builds are 10, 11.25, or 12 twist every single time. Until I build for a boolit shooter LOL!
Then we start talking about 12 14 and 16 twists. Nobody wants the ten.
It just set me to thinking about what's really going on, and lot's of you fellers have been doing this much longer than me.
Fast twist is hard on cast, we all know that. Not impossible, but harder.
I'm just sitting here stroking my chin and thinking
faster is better. It's always better, right?
Hmmmmm. Not with cast.
Why?
Well it could be two things. Either there is something inherently wrong with a cast lead projectile that makes it pee on it's leg when you spin it very fast, or the boolit is getting shredded by the rifling as it's slammed in there. I personally think it's the latter, but I have no proof whatsoever.
I've been devising plans to test the effect of rifling of varying twists on the same boolit fired from the same action but with different twist barrels, but the problem I have is catching the projectiles as they exit the barrel, and doing so in a way that's gentle enough to not damage them so that they can be observed under magnification etc etc etc.
Not an easy thing, especially when here in Arkansas we have mud.....not snow.
I just wanted to hear know if anybody else noticed that according to the calculators, jacketed bullet rifles should be able to run perfectly fine at cast boolit twist rates. and in fact cast boolits should be able to run at jacketed bullet speeds except for the engraving problem.

So does twist rate really not matter as much as I thought it did in the grand scheme of things? Are "problems" that I attributed to twist rate, actually more of a barrel harmonics issue?
I'm questioning the information that I have been fed and accepted as gospel, and cast boolits is just the place to engage in that sort of activity.
No hidden agenda. Just thinking about contradictory information that I have seen last year. Or maybe it wasn't contradictory and it was just my perception, but it lead me to question something that I have taken as gospel and that's usually a good thing.

[johnson1942]

speed or feet per sec. is the answer.

[MBTcustom]

I bet a jacketed bullet, of a proper design, will shoot quite well at 2000 fps in a 10 twist barrel. Would I use a 190 gr HPBT? Nope. A 150 flat base should work.

I don't see twist rate and jacketed as being related as much to velocity as it is bullet length. In Highpower a guy shootig just reduced course matches with an AR is fine with a 9 twist and 60 HP bullets. Try that barrel at a full course match and he is screwed at 600 because he can't use 80 gr bullets due to an overly slow twist.

I think much of the issue with cast at higher velocity and with faster twist barrels is more an issue of lack of experimentation than anything. I am sure that if one of the bullet manufacturers decided to get involved and swage or otherwise form very consistent bullets that they would find an answer. They have the money to spend to make it work, we don't.

I think much of the contradictory info you have seen is based as much on ego as "science". People get ideas in their head and dig in their heels, it get tough to overcome the mental inertia they develop.

I would love an open, spirited discussion on shooting cast at a variety of velocities and in a variety of twist rate barrels. Problem is to keep the facts in and egos out. Not gonna happen. We are all the worse for it.

Great concepts Tim, it is the reality that bites us in the butt.

I think you've got the right of it.
Still think it's worth a shot. After everything that I was taught, told, observed, and studied over the last few months, I really believe I have an open mind on the subject. All I know is that there is very wrong info out there and it's being taught mainstream. Which pieces are superstitious BS, and which are the real deal? Still trying to figure that out.

[geargnasher]

Overstabilization is rubbish. Overspinning a defective projectile, of ANY flavor, will cause large group dispersions. The moot concept of "overstabilization" and "spinning a defective projectile beyond it's 'RPM Threshold'" are two very different scenarios. So you either compromise twist rate and/or velocity to match your projectile quality, or fix the quality issues and shoot them however fast you like. Cast have weaker skins than jacketed and require special attention to the engraving portion of the firing cycle. Beyond that, there is very little consideration needed vs. jacketed as far as the rest of the trip through the gun and to the target goes.

Nobody wants a custom barrel in 10-twist because they don't know how to make, load, or launch the boolits correctly, and instead blame "overstabilization" or (the true cause) boolits that are defective in flight (defective for a multitude of reasons), and they then change barrels to a slower twist and/or velocity to compensate. A heavy cast boolit needs the same amount (about) of minimum rotational velocity as an equal BC jacketed one does. Yes, you can cheat some and run the ragged edge of barely stable, but that's not very reliable, especially way downrange where the velocity is falling off fast.

So, Tim, I'm glad you finally are starting to get an inkling of WHY it is that some of us laugh so hard when a 16" twist .30-'06 barrel is mentioned for shooting high velocity. Truth is, you NEED a 12 at least with 170-ish max bullets, and really could do with an 11. For 200-grainers, you NEED a ten-twist. Why do you think the .300 AAC guys are going to an 8-twist with 240-grain bullets even for subsonic loads?

The RPM threshold theory only applies to lousy boolits. Lousy how they're made, or lousy how they're launched, or both. Like John observed, you can spin a light boolit faster than necessary and experience negative effects from the extra spin due to boolit defects, and that could be considered 'overstabilization', but isn't really. It's simply that the RPM threshold of that system was exceeded by the extra, unnecessary spin. To my knowledge, there is no such thing as "overstabilizing" a true projectile. There IS such thing, however as UNDERstabilizing a projectile.

Gear

[Love Life]

A) Fat Lady was Sub MOA at all distances, and half MOA most of the time depending on how many smokes I had or coffee I drank between shot strings.
B) Fat Lady with the 1 in 7.5 shredded the 68 gr FBHP Match bullets when I juiced it with max charges
C) You need a solid test bed to figure this out...which means somebody needs to open the wallet. What good does it do to get info on this from people who are using different alloys, pots, moulds? People who may have poor or amazing loading techniques? People who may cast poor or amazing bullets (on the outside)? None. Look at your paper patch guys who are swaging and patching. I'd also peek into the BPCR world as they shoot far and accurately. Of course they aren't shooting barn burners...

With jacketed, you can say that length is the important factor...along with how much juice you put behind it.

It has been mentioned several times about the issues with internal voids. I would start with the elimination of that variable and move on from there. Until you can eliminate the issue of internal voids, it'll all be for naught and you'll pull your hair out.

If one person swaged lead (or agreed upon alloy) bullets for ALL in the research to shoot, then that would help immensely.

I would start here:
◦Ask a Question
◦Do Background Research
◦Construct a Hypothesis
◦Test Your Hypothesis by Doing an Experiment
◦Analyze Your Data and Draw a Conclusion
◦Communicate Your Results

Make one thing as consistent as possible (the bullet) to use as your control. Then start attacking the variables.