Boolit Stability, Twist Rate, Velocity and boolit Shape

[303Guy]

It seems to be a commonly idea held that boolit stability increases as velocity increases for a given twist rate. However, this is not necessarily so, specially with cast boolits. Even with spitzer or spire point j's (or cast) the effect can be as much as 65% depending on the form. But for round nose and flat nose boolits the effect is somewhat reversed. Some boolit forms seem to be less affected by transonic velocities but I'm not sure how much to trust my source on that one.

My source is Border Barrel, http://www.border-barrels.com/barrel_twist.htm . Please feel free to try it for yourselves. It might explain why sometimes we get key-holing.

78511

These are two boolit forms with fairly close weight. The top one is a diameter length spitzer and the second a wide meplat boolit.

7850778508
7850978510

[44man]

I have never found that so. I am inclined to consider drive length instead of boolit length. Each boolit needs just one velocity and twist rate changes the velocity.
The only thing that counts for me is to have enough spin so when a boolit goes subsonic, it still has stability. So the question is; what about a boolit that starts subsonic? How about one that stays supersonic all the way?
In truth a transition from supersonic to subsonic should not change stability if spin is correct.

[rsterne]

As an airgunner, I deal in subsonic flight almost exclusively.... I think you will find that for most bullet shapes the minimum stability occurs at a couple of fps under the speed of sound.... The exception is a cylinder (flying ashcan) where the stability continues to drop as the velocity decreases.... Bullets with a very short nose and wide meplat can have a nearly constant stability at nearly all speeds subsonic, but you virtually have to design a bullet to act that way....

Bob

[Crusty Deary Ol'Coot]

I believe that it is in Veral Smith's book, "Jacketed Performance With Cast Bullets" where speaking of effectiveness in taking game, he says that the full wad cutter, similar to the "flying Ashcan" rsterne speaks of in the last post is about as good as it gets for putting game down. The problem was stability at increased - beyond typical handgun target distances - ranges.

I believe that is the reason for the development of the Wide Flat Nose (WFN) and Long Flat Nose (LFN) profiles.

Design a bullet with better stability while retaining a meplat as large as possible.

Seems if my results are any indication, that they came really close to getting it done.

Crusty Deary Ol'Coot

[303Guy]

This for a long cylinder projectile. It indeed does not suffer at transonic but subsonic stability drops off rapidly (muzzle velocity, not down range velocity) just as rsterne states. So there is indication that the software is good. How do cylinder projectiles behave down range?

78550

Could you give me the real dimensions of a wadcutter so I can punch it in and see how it relates to the real world?

[rsterne]

Airgun wadcutter pellets (or pointed, or domed for that matter) usually have a "waist" ie they are thinner in the middle that at the head or skirt.... You can't input that data into the Border Barrels calculator.... I use it all the time for cast bullets, and have found it works extremely well.... I have also input the dimensions for airgun pellets, just ignoring the thin mid-section, but I make an adjustment by reducing the SG of the lead to 10.0 as a "guesstimate" to allow for the lower rotational moment of inertia of a diablo (shuttlecock) shaped pellet.... We recently made a custom barrel for the new JSB .30 cal pellets, using the Border Barrels calculator to determine that a 26" twist was optimum.... and it shoots MOA out to 100 yards.... so I would say our method works just fine....

Bob

[303Guy]

This is for a round nose i.e. the nose is a hemisphere. Note that is it more stable at subsonic muzzle velocities but suffers instability at transonic. I think that effect is borne out by 22lr's being less accurate past the transition range. Mine certainly went south between 100yds and 110yds but it did have a loose muzzle end bore. Cutting it back seems to have cured that.

78557

It seems to be the flat nose that becomes slightly less stable at higher velocity although all boolits are most stable just above transonic and drop off steeply up to about 1500fps.

Playing around with this program leads me to think there is a sweet spot in stability for each boolit style so perhaps the trick would be to select a velocity sweet spot to suite each boolit. Of course there is going to be a wide overlap there. It does seem that for a subsonic FN, the sweet spot might be around 1050 fps. Another sweet spot might be around 1250 fps for FN and RN. A RN might have a sweet spot around 900 fps. Has anyone found this to be so?

I wonder also whether the limiting accurate velocity is due to the reducing stability at higher velocity. HN and FN behave the same.

[rsterne]

The "sweet spot" can be varied with the ratio of length to twist rate.... or conversely, yes you will find a velocity where the stability is around 1.5 (optimum) providing the twist rate is about right for the length....

Bob

[44man]

I don't believe it is just the nose shape. I can give you my best velocities for my WLN and WFN boolits in revolvers. Any less or more opens groups.
My .44 with 310 to 330 gr WLN is best at 1316 fps. The .475 with a 420 gr WFN is best at 1350 fps. the 317 gr WLN in my 45-70 likes 1630 fps. The 440 gr WFN from my .500 JRH shoots best at 1350 fps.
All are accurate to 500 meters, as far as I have shot them. I have no idea the transition point.
As I go from a WLN to a WFN near the same weights, the powder charge stays the same.
If I were to go to a full wad cutter, the bearing length will get longer and need different. I can't tell you because I don't like boolit steerage at the cone. That alone can mess up a wad cutter and shorten range. You need to add boolit damage.
Taking some of my boolits and down loading to match what guys want to do because of recoil, had targets sprayed like a shotgun. I was lucky to keep them on a 3'x3' cardboard at 50 yards.
Going to the 400 plus weight in the .44 and loading max had boolits starting to hit sideways at 50 yards. I could not spin them up.
A boolit must be tested over the whole velocity range for accuracy. I don't think any chart will work.
Even the .22 pistol can stay stable. I figured 200 yard drop at 53", put a target there and hit a steel chicken every shot. I have not found nose shape like a WFN loses accuracy at any range, just has more drop.
With my 45-70 BFR I can hit the ram at 500 meters every shot but with the Ultra Dot I need to aim at a tree branch around 26' over the steel.
I can NOT do that with any Keith style because the shoulders get wiped at the cone.
Then using other revolvers with slower twists has proven lighter boolits are needed. The Freedom .475 shoots best with around 350 gr while my BFR just starts at 400 gr.
The 45-70 BFR has a 1 in 14" twist. It will shoot a huge range of boolit weights with very little change in the powder charges.
Faster twists are easier to work with and extends the range of boolit weights.
It seems what you came up with from the chart will be much too slow. Over spin will extend range.

[303Guy]

I was suggesting that by looking at the predicted stability chart for a given boolit one might determine a good starting velocity or at worst, determine a velocity range to stay clear of. I do notice that velocity ranges that have worked for me are in stable zones on the generated chart. But I would have to do a lot of testing to prove or disprove that idea and I'm not set up for it - yet. I have noticed one boolit appearing to be accurate up to about 40yds then rapidly going wild but that was due to other factors, I'm sure but the mechanism could be aerodynamic instability due to damage in the bore and paper patches coming off unevenly or even nose slump.

[44man]

Boolit damage can be a big factor and never seems to be thought of.
Lee came out with the pointy boolit for 45-70 BPCR and everyone that tried it quit using it, me too. It was slumping into something else. Off center too.

[fecmech]

It does seem that for a subsonic FN, the sweet spot might be around 1050 fps.

In one particular instance I have found that not to be so. Using an early version of Lee's 158 gr FP in my Rossi .357 rifle with a 30" twist. I used a load that was very accurate in my Winchester 16" twist of 1050FPS giving me 100 yd groups of 2-3". At 50 yds out of the Rossi it was basically a 1-1.5" load, at 100 yds it would not stay on a 2' X 4' backer and was tumbling. If I'm reading the chart correctly that bullet should have a stability factor of about 1.2-1.3 which is not optimum but according to the software should work ok. The 16" twist shows a S.F. of 4.3 in that range and it works well. Do I take from this that that particular bullet needs a minimum SF of 4.3??
http://www.border-barrels.com/images/789298.png

[303Guy]

That's a good question. That program uses a SF of 1.5 but allows an optional twist selection in which case two graphs are shown, one showing the required twist and the other the SF at the chosen twist. The software "should" take into account actual boolit shape which it does seem to. What it does not do is allow for a semi-wadcutter style boolit with a step between nose and body. It's quite possible that the 30" twist is too slow for that boolit at that speed.

I loaded for a 357 Rossi with Lee 148gr semi-wadcutter and with its open sights I got a vertical group about 1.5" high and two boolit diameter wide at as close to 100m as we could pace out (I was pretty good as pacing out distance). I put the vertical spread down to sunlight reflection off the front sight making it difficult to see the sight hight. I ruled out velocity variation, I forget how. I may have chronographed the load but I doubt it. Velocity would have been around 1500 fps. I still have that mold and a few recent castings, so I'll punch it in as best I can.

Here is is;
78717

That shows the 148gr semi-wadcutter boolit will not be stable at 1050 fps from a 30 twist Rossi.

[303Guy]

This is for an approximate 180gr 357 boolit in a 30" twist.

78721

This would undoubtedly tumble at 1050 fps but be quite stable above supersonic.

These are not accurate stability calculations as GG's and semi-wadcutter forms cannot be entered. It does show how a boolit could tumble or yaw wildly at subsonic velocities. Border Barrels consider a SF of 1.5 to be optimum.

[Slow Elk 45/70]

Yes I'm confused , worse now than before....must be an old guy thing.. I used to take a boolit that had proved accurate by others and work the powder charges and other factors until I found what worked, or would not work.....part of the fun with boolits. IMHO.....:groner:

[Crusty Deary Ol'Coot]

Makes that two of us, Slow ELK. ---------------------- Maybe if I go to the web site and do some reading it might help, but I get along fine without it at this point. ----------- Probably an Ol'Coot thing, If it ain't broke don't fix it OR My minds made up don't confuse me with the facts :smile: ------- Crusty Deary Ol'Coot

[303Guy]

Crusty is right, if'n it ain't broke, don't fix it! But it could save someone from buying the inappropriate mold.

What motivated me to do this thread is we do get the question from time to time about boolits key-holing and also that the boolit needs to be driven faster to stabilize it. Well, this calculator shows what's happening and why. We also hear of sweet spots and that too becomes clearer (which appears to be boolit design dependent to some degree). So, I thought it might be useful for folks having accuracy issues to have a look at the program. Also useful to gain a better understanding of boolit flight characteristics should one be interested. I dare say if I could spend more time out there shooting I wouldn't want to spend less time studying boolit flight characteristics. I have discovered not to try some boolits at subsonic velocity and not to make some boolit shapes too long. Now that's time saving to me.


I used to take a boolit that had proved accurate by others and work the powder charges and other factors until I found what worked, or would not work.....part of the fun with boolits.Absolutely! Nothing wrong with that approach.

[Wayne S]

Yes I'm confused , worse now than before....must be an old guy thing.. I used to take a boolit that had proved accurate by others and work the powder charges and other factors until I found what worked, or would not work.....part of the fun with boolits. IMHO.....:groner:
"DITTO" but sometimes a design comes along that just looks like it will "shoot" ie; the 311365 ,sadly, so far it hasn't and always takes last place to more conventional designs like the 311299

[Crusty Deary Ol'Coot]

Yep, 303guy, Not in any way discounting the info you folks have found. Other that is except for the fact that I'm just not into that depth of research once I have found what works for me. Many folk spend much time in testing and research looking for a better wheel and we all benefit from any positive results they may come buy. That would be the reason what I shoot does so well for me. Others that have gone before have done the work and I enjoy the benefits. Then with the fact that for hunting firearms, I am pretty much a one bullet/load for one gun sort of guy. Once I find a good load for EVERYTHING that fire arm will be called on to hunt in the way of game animals, well then I'm back to the "if it ain't broke don't fix it mode." :lol: If for example the firearm may be used for the range of critters such as whitetail deer to moose or larger, I don't use one bullet/load for the deer and another for a moose. If the bullet/load isn't up to Moose, then it doesn't get used for deer. -----Crusty Deary Ol'Coot

[44man]

Every formula you find that mentions twist is sure to based on Greenhill.
If you hunt the formula enough you will find a match somewhere for something. Many "experts" change the figures to match their gun and bullet and try to convince others in gun rags that they figured it out.
Not going to happen---EVER!

[303Guy]

They sure seem to. Many mention Greenhill. The Border Barrels calculator doesn't use Greenhill. It takes aerodynamics into account, basically using the tipping moment caused by form and velocity. Greenhill does not take boolit form or velocity into account.

[MUSTANG]

303Guy:

Using the Barrel Twist calculator, how do we account for the difference in real world velocity/accuracy performance between a cast boolit and a jacketed with similar profiles at given velocities? I went to the site and see that you can adjust for various projectile specific gravity and jacket-no jacket.


Mustang

[rsterne]

The thing about the Border Barrels Calculator that is different is that the mathmatics it is based on allow computing data for subsonic velocities.... I have never seen that anywhere else, and particularly not in an easy to use calulator format.... It does a particularly good job of showing the instability (or reduction in stability) that occurs at the instant a bullet drops below the speed of sound....

Bob

[303Guy]

What it doesn't do is allow for grease grooves and a stepped nose. Also no allowance for hollow nose - that must be treated as a flat nose for a close approximation I suppose one could say. I can't answer for the differences in real world performances. I would hazard a guess that the lower pressure typical of cast boolit loads would reduce barrel vibration which would improve accuracy potential. Then again it is harder to make as perfect a boolit as a jacketed. I would say though that the more dense material of the cast would be in the favour of cast except they have surface perturbations which would surely reduce stability and also overall density. I just don't know.

[rsterne]

I think the thing to remember is that if you start playing around with and SF (stability factor) down close to 1.0.... sooner or later you will see the bullet tumbling.... The military use an SF of around 2.0 for good reason.... Under 1.5 you're pushing the envelope.... Over 4.0 likewise, from overspin, which will emphasize any imperfections in the bullet.... Use a twist that puts you in the happy zone around SF 2.0 and you'll never have a problem.... Before more sophisticated programs became available we just used Greenhill, and never had a problem....

Bob

[44man]

Not so, Greenhill is the worst of the worst. It was for BP cannons and maybe not good even for them.
The predicted twist rate is ALWAYS too slow for any gun.
Now talk about a FN boolit with grease grooves. Do the grease grooves affect anything? Heck no, the wave from the nose keeps air from the sides of the boolit. Might also with a pointy boolit since shock waves are spread from the nose.

[303Guy]

Do the grease grooves affect anything? Heck no, the wave from the nose keeps air from the sides of the boolit.I wondered about that but found shadow graph's of boolits in flight and there are distinct shock-waves formed along the boolit length.

79123 Supersonic Bullet (or so it says?)

[303Guy]

Here's some of what goes into boolit stability calculations;

79129

Here's the link;

http://www.nennstiel-ruprecht.de/bullfly/stab.htm#Dynamic_stability

[303Guy]

A 22lr boolit. Note the smaller shock-wave propagating from the ogive step.

79133

A 7.62 NATO bullet. Note the shock-wave propagating from the crimp groove.

79134

I'm not sure that the presence of a shock-wave means the air-flow is having any effect. Hornady show the same ballistic coefficient for two otherwise identical bullets apart from one with a cannelure, if that means anything.

Anyway, I thought these shadowgraphs were interesting.

[Oreo]

The lube groove does make a significant difference in the center of mass calculation.

[44man]

I wondered about that but found shadow graph's of boolits in flight and there are distinct shock-waves formed along the boolit length.

79123 Supersonic Bullet (or so it says?)
True, always turbulence. But the main wave from the nose has the most affect as does the crazy stuff behind the base. Reducing the base turbulence with a boat tail in a rifle adds distance stability but does nothing at close range. Even smooth bullets will have some at the sides.
But to have any turbulence from a GG throw a boolit will not happen.
Those are shock waves, even side to side around the boolit.
Now what if the boolit does not have spin stability and starts to turn against the nose wave? (Slowing gyroscope)
What is not answered with the pictures is how much air is at the boolit sides. It is a low pressure area. Air tries to rush back in but will be even all around.
Like an airplane wing with high pressure under and low above. We don't want that with a boolit.
If you want trouble, shift the nose wave back to the GG. How about to the shoulder on a Keith?
Does the wave move back as the boolit slows? Does it go away.
Those pictures are just past the muzzle, not downrange.
You also see the pressure wave that if a FN is too fast in a deer, it will move tissue away from the boolit. Expansion will destroy the wave because it slows the boolit.
How about the wave from a subsonic boolit? It will taper back more and the shock waves might go away from GG's. We don't know but the slower hard boolit can do more damage in an animal.
I go crazy thinking about this stuff.
What I actually see with dead animals is my teacher and some things turn everything backwards from what is believed or in print.
Right now with hard cast, the .44 and .475 have no equal in revolvers. Every other gun needs an alloy adjustment.

[1Shirt]

Interesting but I am not much into charts and graphs! They sort of bring out the "anti math animal" in me! Sort of like the trial and error method. If I cast it and can't get it to shoot worth a hoot, I have learned from reality.
1Shirt!

[Oreo]

You also see the pressure wave that if a FN is too fast in a deer, it will move tissue away from the boolit. Expansion will destroy the wave because it slows the boolit.

The first sentence is mooted by the second.

[popper]

Curve on #14 - that discontinuity in the curve makes me suspicious. Always has, always will. I put the numbers in for my CBs & rifles. Top curve says over stable, bottom says under stable. In the trans-sonic region, it show over stable, which shouldn't by itself cause tumbling. Yes, the shock waves move around with velocity, but are radial so don't influence the flight appreciably. In the #34 post, note the twist effect on the pressure ridges at the base, ridges are twisted also. Boat tail allows a faster flight. Yes, I looked at the formulas and can see how the discontinuity occurs. I would just white it out.