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

[geargnasher]

That's been done on an individual basis, LL. The problem is that very, very few believe the results or care enough to learn how to do it for themselves. It is far easier to attack than it is to learn, and most people are incapable of effectively following the process you outlined above on their own. Finding the solution to THAT problem is the real challenge that is rarely overcome on this board. I have done it to some extent with regard to high-velocity cast shooting, and proven to myself who is right about what here and who/what to ignore. Until the majority of the can't-dos on this board get an open mind, no cooperative learning will take place on this subject, only fighting. The can-dos are done fighting over it, and pretty much done sharing publicly until this situation changes. It's funny that when I was studying to be an engineer, they never mentioned how the number-one obstacle to progress isn't some sort challenge that a scientific mind has been trained to meet and defeat, it is something else that the scientist is woefully ill-equipped to handle: .......... it's called politics.

Gear

[AlaskanGuy]

I would think that the the faster the twist, the more it digs into the fps.... So wouldn't that mean that you would have to push harder to get the same fps in something that has a faster twist?

[Mk42gunner]

I do not know the answer, but I do have a few comments/ ideas/ questions.

-IIRC the Greenhill formula was originally worked out for either artillery or Naval gunfire, not small arms. It has just been adjusted a bit to sort of work. I can remember different sources using 150 or 180 for constants.

-The ten inch twist for the .30-06 came about because the Army decided on a ten inch twist for the 220 gr Krag projectile @ ~2000fps. When they came up with the .30-03 they used the same projectile at about 23-2400fps. When they went with the 150, they shortened and rechambered the barrels by one thread, to save money.

I can't be the only one that read of the lot of 03 Springfield's that showed better than average accuracy; which was traced to a rifling machine cutting a slower twist, (1:13" runs through my mind). The rifles were rebarreled with 1:10 and the others destroyed.

The 1:10 stayed in the Gov't .30 caliber rifles all the way through the M1 Garand; not sure of the Johnson, but I bet it was 1:10 too. It wasn't until the M-14 and 7.62mm NATO came along that they went to 1:12 twist.

-The talk of imperfect cast boolits has merit. Does anyone have access to one of the Juenke bullet testers? From what I have read they can find even slight differences in jacketed match bullets such as Berger and SMK. Might be worth checking into.

-And as Gear said:

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.

I will add that you can overspin jacketed bullets, I have seen it using M195 and M855 in the same rifles.

Robert

[MBTcustom]

I do not know the answer, but I do have a few comments/ ideas/ questions.

-IIRC the Greenhill formula was originally worked out for either artillery or Naval gunfire, not small arms. It has just been adjusted a bit to sort of work. I can remember different sources using 150 or 180 for constants.

-The ten inch twist for the .30-06 came about because the Army decided on a ten inch twist for the 220 gr Krag projectile @ ~2000fps. When they came up with the .30-03 they used the same projectile at about 23-2400fps. When they went with the 150, they shortened and rechambered the barrels by one thread, to save money.

I can't be the only one that read of the lot of 03 Springfield's that showed better than average accuracy; which was traced to a rifling machine cutting a slower twist, (1:13" runs through my mind). The rifles were rebarreled with 1:10 and the others destroyed.

The 1:10 stayed in the Gov't .30 caliber rifles all the way through the M1 Garand; not sure of the Johnson, but I bet it was 1:10 too. It wasn't until the M-14 and 7.62mm NATO came along that they went to 1:12 twist.

-The talk of imperfect cast boolits has merit. Does anyone have access to one of the Juenke bullet testers? From what I have read they can find even slight differences in jacketed match bullets such as Berger and SMK. Might be worth checking into.

-And as Gear said:


I will add that you can overspin jacketed bullets, I have seen it using M195 and M855 in the same rifles.

Robert

Indeed! Larry Gibson made mention of that very piece of history on the phone to me, and that appears to be fact.

The guys shooting lighter 150 grain boolits ask for 10 twist, but that is still a touch too fast. Seems that for jacketed, running at 2600-2800 FPS with 168-175 grain bullets, that 11.25 twist is the hells bells.

[MBTcustom]

Barrel twist is a variable I have control over Brad.
I care very much about chamber design, boolit quality and fit, precision of the loads etc.
A brief glance at the stickies that I have written will prove that definitively (at least that I care, not necessarily that I am right)
In this thread, I am exploring twist rates as they pertain to cast lead boolits that are assumed to have been cast, fit, and loaded perfectly.
I'm not missing the forest for the trees. I'm systematically considering every factor......obviously, and I am drawing conclusions.......in public.

Just trying to further the general understanding of cast boolits and bring us to a point that we are considering things that most take for granted, or just work around.

Now granted, barrel twist is a muddy soup and the science vs. superstition definitly seems lop sided towards the ethereal rather than the real.

As I have mentioned on the phone to you, I am looking to devise a few controlled tests using different twist barrels with that being the only differing factor. I want to know how this stuff works.
I have many theories, and opinions, but my thoughts are no more grounded in reality that anybody elses. It's theory based on loose pieces of disorganized data. I want to fix this, but I'm not made of money nor time, so I'm hoping that through this discussion, I can devise a few tests that will render a few pieces of better informed data.
Heck, it's all for fun. This is my hobby, slightly different than my vocation.

[243winxb]

http://www.bergerbullets.com/litz/TwistRuleAltWP.php

Since the onset of BC
variations seems to be consistently at or below Sg =
1.25, keeping the predicted stability above Sg = 1.30
is probably ample margin for error.
One might be tempted to try and design a rifle and
bullet combination to realize the increased BC of the
sweet spot observed near Sg = 1.23. The authors
think this is a bad plan and unworkable approach.
Even if the muzzle velocity can be held sufficiently
constant, Sg will change with environmental
conditions and shift the Sg out of the sweet spot. In
addition, the larger shot to shot variations in drag will
negatively impact long range accuracy and more
than erase the marginal gains from an average
increase in ballistic coefficient.

http://arxiv.org/ftp/arxiv/papers/1309/1309.5039.pdf

[MBTcustom]

http://www.bergerbullets.com/litz/TwistRuleAltWP.php http://arxiv.org/ftp/arxiv/papers/1309/1309.5039.pdf

Great link 243winxb!

Look at what is written there:

Bullet companies usually publish a minimum
recommended twist rate, usually a twist that will
ensure bullet stability under the most dense
atmospheric conditions a hunter or recreational
shooter is likely to encounter, with some margin of
error so that no shooter is likely to ever report
evidence of bullet tumbling as long as the minimum
twist rate is used.

It seems reasonable to assume that our cast lead boolits are much more in need of the stabilizing benefits of faster, not slower twist rates!
Compared to a SMK, a cast lead boolit looks like a flying sack of potatoes no matter how well it was cast.

We are less aerodynamic. We have less BC. We have less SG. We need twist (theoretically).

If I could rewrite the way things are done, it would almost seem that highly aerodynamic jacketed bullets should be spun slower, and our boolits would be the ones getting the faster twists.
In the words of Spok "this **** aint logical".

It almost seems like we have thrown physics out the window because it's hard to get a boolit out of a rifled barrel unharmed eh?

[geargnasher]

No need to reinvent the wheel, Tim. Use existing data for the BC and projectile weight to determine minimum twist rate for velocity and distance desired, add a margin for variables, and accomplish the rest at the reloading bench. This has all been done before. What HASN'T been done is a comprehensive explanation of how to do this last part, the part at the reloading bench, which is by far the MOST important factor to achieving accuracy at high velocity with cast in any rifle.

Gear

[MBTcustom]

No need to reinvent the wheel, Tim. Use existing data for the BC and projectile weight to determine minimum twist rate for velocity and distance desired, add a margin for variables, and accomplish the rest at the reloading bench. This has all been done before. What HASN'T been done is a comprehensive explanation of how to do this last part, the part at the reloading bench, which is by far the MOST important factor to achieving accuracy at high velocity with cast in any rifle.

Gear

I hear ya Ian.
But I don't think that lots of people here are thinking about such things, and I wanted to give food for thought. Well, and I was hoping I might learn something in the process as well.
Muzzle not the ox don'tcha know.
If we can have a 40 page thread on thread killers or sarcasm font's, surely we can discuss twist rates for a minute eh?

[Whiterabbit]

No need to reinvent the wheel, Tim. Use existing data for the BC and projectile weight to determine minimum twist rate for velocity and distance desired, add a margin for variables, and accomplish the rest at the reloading bench. This has all been done before. What HASN'T been done is a comprehensive explanation of how to do this last part, the part at the reloading bench, which is by far the MOST important factor to achieving accuracy at high velocity with cast in any rifle.

Gear

What equation to use? Surely not greenhill? I can plug that in for 45/70 and get stability numbers in the 6+ or even double digits when supposedly 1.5 or less is best?

[MT Gianni]

While you can control the twist Tim, you cannot control the different alloys, 150 F swings in pot temps, lack of regular fluxing, dipper vs bottom pour, quenched vs tossed on a towel, improperly closed molds, etc. You cannot control the out of round sizer dies, dirty molds, voids visible or internal and our of round loading dies/press's and process's that make up much of our board. Far too many want to know what is the cheapest mold, lubing process and dies so I can shoot cheap lead with the same accuracy as my 5.56 match bullets in my existing gun. We can control twist but not what twist does to improper boolits as it will effect them in different ways. Thanks for making me think tho, I do know that at 59 & 5/6 my eyes best days are in the past.
I think too much is made of speed in the shooting sports and not enough of range estimation, holdover and sighting techniques. I know I don't need 3300 fps to kill a ground squirrel but that hydro-static shock is impressive.

[Larry Gibson]

geargnasher is absolutely correct with; " 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."

The twist required for proper stabilization for best accuracy (each length of bullet will have an "ideal" twist rate for best accuracy) has absolutely nothing to do with the adverse affect RPM has on any imbalanced bullet. An imbalanced bullet may be horribly inaccurate yet fully stabilized. As I've stated numerous times before; the RPM threshold of cast bullets has nothing to do with stabilization of the bullet in flight.

Ideally for any bullet of any flavor the minimal twist required for full stabilization will produce the best accuracy for that bullet all other things being equal. The reason being is the adverse affect of any imbalance will be minimized by the minimal RPM produced. Hence the reason for the 13 and 14" twist in Palma rifles which were originally intended for use with 7.62 NATO ball ammunition. The bullets of such ball ammunition was and is not noted for their match qualities. The minimal twist required for stabilization of the 145 - 155 gr bullets at the higher velocities the 27 - 30" barrels offered much lowered the RPM which minimized the adverse effect the imbalances in the M80 bullets had on accuracy, especially since the targets were 800, 900 and 1000 yards away.

Larry Gibson

[Pb2au]

It's funny that when I was studying to be an engineer, they never mentioned how the number-one obstacle to progress isn't some sort challenge that a scientific mind has been trained to meet and defeat, it is something else that the scientist is woefully ill-equipped to handle: .......... it's called politics.

Gear

That Sir, is a sad true story.
The engineer I have worked with for the last 8 years told me the same thing in a different way.
"Those that will benefit the most from change in a manufacturing process will be the ones that oppose it the most."
That has stuck with me a very long time. For the life of me, I do not understand when I read something here, like "hey, I just shot an awesome group with my rifle, at 2700 fps, here is the picture of the group", it turns into a blood bath. I can understand why folks get tired of rocks being thrown at them.

geargnasher for one, and others I have read here always come back again, and again to one single truth. (as I interpret it mind you).
It is a process quality issue. To start walking the path of higher velocities with a cast lead projectile, everything you do up to point where the finished cartridge is in battery and the trigger is pulled, has to be the best work you can do. Best alloy, best design of projectile, best sizing, best loading practice, best load development. The end game is to arrive to this point of quality, you have to identify and control your variables.
The lead projectile simply in my mind isn't as forgiving as a jacketed one. It is a materials issue.(again, in my mind) So those that walk the path gain this understanding and dial down the tolerances on everything else to account for it.
Am I dialed down this tight on my current Mauser project, or my Krag project, no. Not yet....but, this is how we learn. We learn by doing and paying attention to the details.
So, the conclusion to my long winded ramble is that I see your focus on the twist rate as an important key to this, and it will tease out the other details that create the whole picture. That is to say, it will tease them out for those of us that really read the details.

Tim, this is an excellent experience. I have re-read this thread a few times now as it grows and continue to make notes. Thank you to the contributors.

[MBTcustom]

Ok Larry, im with you so far with the Palma rifle twists etc, but how does it work when we take a heavier, longer and less precise projectile like a boolit, and we shoot it slower and stabilize it less??

I understand damage to the boolit on engraving, blah blah blah, but assuming you could get the boolit out of the barrel undamaged like with paper patch etc, why would you shoot and spin it slower if it's longer ?
Seems that if you have a damaged boolit, it would be counter intuitive to take stability from it by dropping it's speed and RPM to a place that it never would have run correctly in the first place, to apologize for shooting a damaged projectile in the first place.

That just rubs my brain the wrong way for some reason.

Seems that if we build the perfect, concentric, well fitting load, we should either twist it correctly or find a different projectile that fit's our parameters better, or find a way to launch it without damage.

This is all purely theoretical, and I'm assuming there is a way to accomplish launching an undamaged boolit. I have heard claims from people who claim to have done it, and I don't doubt it for a second, being the type of guy that takes a man at his word till he proves himself a liar.
I have never seen, nor experienced this myself personally (at least not to my knowledge).
I'm just saying that if it were possible why would we spin a boolit slower?

[MtGun44]

I have access to a Juenke machine and have tested 2nds of 175 Mkings in some significant quantites.

It is my understanding that it makes no difference how fast you spin a bullet once you have gotten
enough speed to stabilize it gyroscopically - until it blows up from rotational forces. I have actually
blown up some small .22 cal bullets by spinning them too fast. This presumes a PERFECTLY
balanced bullet/boolit.

The real world situation is that ALL bullets/boolits have some degree of imbalance. The development
and use of the Juenke machine has helped bullet makers like Sierra get their products dramatically
better as to balance. Another driving factor was the military developing the (IIRC) M856 Tracer round.
This is a 1200 yd trace 5.56 NATO round and the bullet is extremely long - under arctic conditions it
required a 7 inch twist barrel to stabilize, so the Army standardized on that twist. Most ammo shot
thru the military guns is 62 gr M855, and they need to be well enough balanced to meet accy requirements,
so the goalpost is moved - as far as how good a "normal production bullet" has to be balanced- SINCE THEY
ARE GOING TO BE SPUN WAY FASTER THAN IS "NECESSARY".

Us civilians started getting 7 inch twist barrels and shooting 55 FMJBT M193 and even 50 gr flat base boolits - at
amazingly high rotational speeds. Some varmint bullets blow up, but we EXPECT THE AMMO TO SHOOT WELL,
EVEN WHEN SPUN "TOO FAST" and this is another factor driving "normal production bullets" standard of
balance way up - and the Jeunke machine has allowed bullet makers to measure their quality, along with
range testing in "too fast" twist barrels.

Once we had 80 gr VLDs in .224 diam - people started wanting scaled up versions for .30 cal - and guess
what? They needed fast as heck twist barrels, and then they would be upset if that fast twist barrel
scatters 168 MKs all over the target - another factor driving up the quality of bullets - as far as balance
is concerned. There are now lots of rifles being made with "too fast" twist, so the bullet makers need to
balance their jbullets well enough to avoid inaccuracy - again moving the goalposts on balance quality.

The flies in the ointment is three things: 1) boolits are relatively lower quality (well mine are - maybe yours
are perfect) and can't stand to be spun too fast because they will wobble and not shoot as accurately.
2) boolits are comparatively weak structurally and can blow up from rotational forces at spin rates that
jbullets can survive.
3) boolits can't stand the mechanical shock of spinning up super quickly in fast twist rifling and can strip,
or skid and get totally messed up.

These three things mean that we have to keep our boolits running JUST fast enough rotational rate to stabilize,
but not to fast because of relatively lower quality of balance, rotational structural strength and ability to grab
the rifling when launched. Jbullets - with modern quality levels - can be spun as fast as you want until they
blow up and are still pretty accurate. The old "overstabilized" (IMO) was just a way of saying "spun to fast
for an out of balance bullet". "Overstabilized" doesn't seem to happen very much these days with jbullets -
because they are far better balanced bullets than was standard 30-40 years ago.

Bill

[geargnasher]

geargnasher is absolutely correct with; " 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."

The twist required for proper stabilization for best accuracy (each length of bullet will have an "ideal" twist rate for best accuracy) has absolutely nothing to do with the adverse affect RPM has on any imbalanced bullet. An imbalanced bullet may be horribly inaccurate yet fully stabilized. As I've stated numerous times before; the RPM threshold of cast bullets has nothing to do with stabilization of the bullet in flight.

Ideally for any bullet of any flavor the minimal twist required for full stabilization will produce the best accuracy for that bullet all other things being equal. The reason being is the adverse affect of any imbalance will be minimized by the minimal RPM produced. Hence the reason for the 13 and 14" twist in Palma rifles which were originally intended for use with 7.62 NATO ball ammunition. The bullets of such ball ammunition was and is not noted for their match qualities. The minimal twist required for stabilization of the 145 - 155 gr bullets at the higher velocities the 27 - 30" barrels offered much lowered the RPM which minimized the adverse effect the imbalances in the M80 bullets had on accuracy, especially since the targets were 800, 900 and 1000 yards away.

Larry Gibson

Yes, exactly. Going at the minimum twist required for the intended use (range, weight, bc, etc) buys a little insurance against balance problems with bullets of any flavor. The only issue with this is that, when optimizing twist for the desired application, there is obviously a natural performance and option trade-off with any bullet type or design that falls too far left or right of center.

When extending the twist rate of a barrel to reduce the effect of imperfections in our cast boolits at higher velocities, there is a practical limit to boolit weight. There one has to make a decision on weight, maximum distance, and velocity range for a particular application. The optimum stabilization rpm still applies the same to cast or jacketed with a given twist. If one wants the option of shooting 220-grain cast boolits in their '06, but selects a 14" twist rate optimized for 150-grain or lighter projectiles, there will be stability problems regardless of how well the heavier boolit is launched.

As you pointed out, stability and imbalance (imbalance for whatever reason), are two different animals entirely. If your cast boolits are correct for the twist and velocity but still don't group well, then the problem lies with the balance quality and launch of the boolit, and after that, fine-tuning to the harmonics. All of those things have to be right to get tiny groups.

Gear

[MBTcustom]

Solid discussion fellers! Good stuff, and great info.

So if I understand this right, as long as the RPM's are enough to soundly stabilize the projectile, you're in like Flynn no matter what speed you are shooting at. It falls to harmonic nodes from there.

RPM does in fact have a detrimental effect on imperfect boolits. Since boolits have inherent imperfections built into the process itself, RPM's should be kept to a minimum in order to give the best results.

Jacketed projectiles are able to survive super fast twists accurately because of their superior accuracy of manufacture, not necessarily the hardness of the jacket. Therefore, attention to detail and precision with our cast lead boolits becomes paramount to shooting fast twist, regardless of weather the boolit is damaged in the throat or not. If you got janky boolits, you're gonna have janky groups.

Interesting how a discussion on ideal twist seems to bring all areas of our discipline together.

[geargnasher]

Solid discussion fellers! Good stuff, and great info.

So if I understand this right, as long as the RPM's are enough to soundly stabilize the projectile, you're in like Flynn no matter what speed you are shooting at. Within reason, yes. It falls to harmonic nodes from there. No. There's the big elephant in the room to consider next, THEN the much more minor vibration tuning which is easily managed through a number of fine tuning techniques.

RPM does in fact have a detrimental effect on imperfect boolits. Since boolits have inherent imperfections built into the process itself, RPM's should be kept to a minimum in order to give the best results. That depends on the severity of you "inherent imperfections" and the purpose of the rifle/load. Versatility is compromised if you focus on the minimum and not on the elephant. Mastering the elephant will give you a LOT more options with a rifle.

Jacketed projectiles are able to survive super fast twists accurately because of their superior accuracy of manufacture, not necessarily the hardness of the jacket. No, it's the elephant again. Read Bill's post again. Therefore, attention to detail and precision with our cast lead boolits becomes paramount to shooting fast twist, regardless of weather the boolit is damaged in the throat or not. If you got janky boolits, you're gonna have janky groups. If the boolit is torn up at launch, or even under hard acceleration, it doesn't matter how good it was when it was ready to fire. Likewise, if you achieve a good launch, but the boolit has an internal balance defect or aerodynamic flaw, it won't shoot straight either past whatever it's rpm threshold happens to be. Clear as mud?

Interesting how a discussion on ideal twist seems to bring all areas of our discipline together.

Nobody's talking about the elephant, Tim, which is the BIG deal with getting cast to shoot. The elephant's name is "Launch". But since that doesn't deal with twist rate directly, is a better subject for another thread that I'm afraid won't get much meaningful traffic here.

Gear

[nekshot]

starting to think the savage recievers are perfect for booliteers. Get a barrel/twist for each boolit in its weight class. 3 differant barrels should cover my needs!

[MBTcustom]

I'm really glad I started this thread. I think you could be right Brad. I've been thinking of twist the wrong way......I think. I'm going to reread this a couple more times and sleep on it, but it sure is looking like I need to change my thinking.
I think I was right about the lower end of the twist, ie. there's a point where you're just not stabilizing the boolit properly, but my thinking that there is a narrow range of twist that a boolit will like to run is starting to smell a little funky......like BS.
Oh well, this is why I'm still here.