Bullet stability

[NigelM]

There has been a lot of chat about stability recently. Will this bullet stabilise in this twist barrel etc.

Some are pushing the Berger Stability Calculator as the answer to this question but that's not entirely correct. The Berger calculator is designed for their own bullets, none of which are tipped. Adding a plastic or aluminium tip to a bullet makes a big difference to the stability calculation as the density of plastic is about 1 g/cm3 and lead 11.7 g/cm3. Aluminium fits in at around 2.7 g/cm3. A 0.150" tip on a 1.4" bullet makes a big difference.

The JBM stability calculator is much better than the Berger for anything tipped. It allows you to enter tip length and builds that into the stability calculation. Amazing what difference it makes. This is however for plastic tipped bullets.

For Aluminium tipped bullets Michael Courtney updated a Bryan Litz version of the Miller and Courtney Stability Formula to include a calculation for aluminium tipped bullets. It works out very roughly at 5% less stable than an equivalent plastic tipped bullet. If anyone wants the spreadsheet I'm happy to email it but don't seem to be able to post it here as it's an .xlxs file.

If you put the same 1.400" 130gr .284 bullet with a 0.150" tip in an 9.5 twist rifle at 2900 fps into the 3 different calculators you get the following results:

Berger : 1.11
JBM no tip : 1.114
JBM Plastic tip : 1.374
Modified M&C Plastic tip : 1.374
Modified M&C Aluminium tip : 1.322

The stability factor that is thrown out is a guide, it's not gospel. It's a ratio of the rotational momentum of the spinning bullet vs it's overturning moment. A bullet always wants to drive itself backwards as the rear of the bullet is heavier than the front and the heavier end wants to go first - which is why we have to spin it - the question is how much. A ratio of below 1.0 and the overturning moment tends to win and the bullet tumbles. Greater than 1 and it should fly front first but is likely to wobble a bit, reducing the BC, until it get's to about 1.3 at which point it should be stable until it slows down to around the speed of sound, which is why LR shooters are really looking for a SF of 1.5 or so as that extra stability improves BC as the bullet slows - but were talking well beyond deer ranges. As long as you are scoring over about 1.3 on the JBM calculator you should be stable at all deer ranges.

So if you're using plastic or aluminium tipped bullets and wondering about stability in your rifle beware the Berger calculator. Better going with JBM or even better the modified M&C spreadsheet. They will give you a more accurate result.

 

[Jelen]

That's good to know and i've learnt something too, so thanks for putting that up.

 

[Muir]

Does Berger make a 130 gr x 284 bullet?? I've never seen it. ~Muir

 

[antsa]

There has been a lot of chat about stability recently. Will this bullet stabilise in this twist barrel etc.

Some are pushing the Berger Stability Calculator as the answer to this question but that's not entirely correct. The Berger calculator is designed for their own bullets, none of which are tipped. Adding a plastic or aluminium tip to a bullet makes a big difference to the stability calculation as the density of plastic is about 1 g/cm3 and lead 11.7 g/cm3. Aluminium fits in at around 2.7 g/cm3. A 0.150" tip on a 1.4" bullet makes a big difference.

The JBM stability calculator is much better than the Berger for anything tipped. It allows you to enter tip length and builds that into the stability calculation. Amazing what difference it makes. This is however for plastic tipped bullets.

For Aluminium tipped bullets Michael Courtney updated a Bryan Litz version of the Miller and Courtney Stability Formula to include a calculation for aluminium tipped bullets. It works out very roughly at 5% less stable than an equivalent plastic tipped bullet. If anyone wants the spreadsheet I'm happy to email it but don't seem to be able to post it here as it's an .xlxs file.

If you put the same 1.400" 130gr .284 bullet with a 0.150" tip in an 9.5 twist rifle at 2900 fps into the 3 different calculators you get the following results:

Berger : 1.11
JBM no tip : 1.114
JBM Plastic tip : 1.374
Modified M&C Plastic tip : 1.374
Modified M&C Aluminium tip : 1.322

The stability factor that is thrown out is a guide, it's not gospel. It's a ratio of the rotational momentum of the spinning bullet vs it's overturning moment. A bullet always wants to drive itself backwards as the rear of the bullet is heavier than the front and the heavier end wants to go first - which is why we have to spin it - the question is how much. A ratio of below 1.0 and the overturning moment tends to win and the bullet tumbles. Greater than 1 and it should fly front first but is likely to wobble a bit, reducing the BC, until it get's to about 1.3 at which point it should be stable until it slows down to around the speed of sound, which is why LR shooters are really looking for a SF of 1.5 or so as that extra stability improves BC as the bullet slows - but were talking well beyond deer ranges. As long as you are scoring over about 1.3 on the JBM calculator you should be stable at all deer ranges.

So if you're using plastic or aluminium tipped bullets and wondering about stability in your rifle beware the Berger calculator. Better going with JBM or even better the modified M&C spreadsheet. They will give you a more accurate result.

+1
If you want some background do a Google search for the Miller Courtney Stability Formula. Couple of papers describing the work behind it are out there.

Cheers

 

[Sharpie]

There has been a lot of chat about stability recently. Will this bullet stabilise in this twist barrel etc.

This is all very well, but when the bullet manufacturers won't even publish how long their bullets are, with or without tips, never mind some sort of BC, frankly we are left guessing. Unless we actually have some in our hands to at least measure for ourselves, then put to the test.

In reductio ad absurdum a round ball ought to shoot dead straight from a smoothbore, nothing preventing that e.g. wanting to tumble. Which they seemed to do quite effectively back in the day.

And yet once rifling was introduced they were found to benefit from a little (not very much) rotation. I mean say a 60" twist at muzzle loader velocities is hardly much of a spin, but undoubtedly beneficial.

Likewise a round ball ought to be the least likely thing to be disrupted when moving between supersonic and transonic, and yet they still do seem to. depending on who you believe.

Making a lathe turned thing, then wanting to put a tip on it to improve aerodynamics,/BC whatever is probably easiest if you go with an aluminium one, Because a lighter polymer one requires injection moulding tooling, and a knowledge, and supply, of suitable engineering polymers. Magnesium alloy might be better, or even titanium, but that's probably wishful thinking. Nerthelesss a tip is an extra part that needs to be very precisely aligned with the rest of the bullet, if it's going to be consistent.

Maybe why the very best target bullets still don't have tips on them.

Unless that tip is also somehow part of a controlled expansion mechanism, I dunno, that needs serious understanding of terminal ballistics in the real world, not just gelatine and water etc.

Besides, apart from very long range target shooting, I don't expect my bullets to slow down enough to get into the transonic region, certainly not at any conceivable (to me) hunting ranges.

Want to shoot out to 1000 metres, a mile or even more, well, that's a quite different matter. F-class etc. Never tried that, but mucking about at up to 900m used to be a regular thing for me. Against paper, marked up, not just gongs. Fun though gongs are, I always feel a little tawdry playing with them.

So, to me, an aluminium tipped bullet, to be used for hunting, seems an oddity. If you want something for extreme long range targets, then why not just make the whole monolithic thing in one piece. Where the use of such is permitted.

Sorry, just my rambling thoughts.

 

[NigelM]

The OP is just pointing out that if you want an accurate SF to decide whether or not to try a specific tipped bullet in your rifle the Berger calculator should not be used. JBM for plastic tips or if aluminium tipped the modified M&C spreadsheet. I’m not arguing the rights or wrongs of any specific bullet type or design, or indeed the way the makers behave and the data they publish.

As an aside, JBM have a great deal of data on bullet length. Always worth looking there first if your manufacturer is not publishing the information.

 

[Bavarianbrit]

Any links to JBM?

 

[Jelen]

JBM ballistics link: calculators.shtml

 

[NigelM]

Just to shake the tree a bit harder on stability, there are a number of questions I still have which are unanswered.

We all presume the stability calculators are right. After all some very clever mathematicians have written them. However, there are still anomalies and as I said the number is more a guide than gospel.

The calculators were I believe originally written for lead cored jacketed bullets - even artillery shells. With the advent of copper bullets with significantly lower density than lead does the calculation need to be tweaked a little to make the formula more accurate for copper or is density irrelevant? I know the formula doesn't have an input for density but should it - mathematically?

Also, the new lathe turned mono metal copper bullets are being built to tolerances of 2 microns. That means the central axis of rotation is exactly in the middle - well it might be 1 micron out. Compare this to lead core jacketed bullet with a variance of jacket thickness and a variance in now the lead settles within that jacket as well as the accuracy of the bullet dimensions, the central axis of rotation is likely to be many microns out. How does this difference in build accuracy effect the SF ratio number that's acceptable for good stability - perhaps good stability on a good lathe turned bullet is achieved at 1.2 rather than 1.3.

Lots to think about if you don't sleep much at night...

 

[User00033]

Just to shake the tree a bit harder on stability, there are a number of questions I still have which are unanswered.

We all presume the stability calculators are right. After all some very clever mathematicians have written them. However, there are still anomalies and as I said the number is more a guide than gospel.

The calculators were I believe originally written for lead cored jacketed bullets - even artillery shells. With the advent of copper bullets with significantly lower density than lead does the calculation need to be tweaked a little to make the formula more accurate for copper or is density irrelevant? I know the formula doesn't have an input for density but should it - mathematically?

Also, the new lathe turned mono metal copper bullets are being built to tolerances of 2 microns. That means the central axis of rotation is exactly in the middle - well it might be 1 micron out. Compare this to lead core jacketed bullet with a variance of jacket thickness and a variance in now the lead settles within that jacket as well as the accuracy of the bullet dimensions, the central axis of rotation is likely to be many microns out. How does this difference in build accuracy effect the SF ratio number that's acceptable for good stability - perhaps good stability on a good lathe turned bullet is achieved at 1.2 rather than 1.3.

Lots to think about if you don't sleep much at night...

You and Sharpie are bosum buddies Nigel, you just don’t know it yet.

I remember arguing on here with the usual suspects (Miki?) and having to prove that Miller updated his original calcs to include the effect of the tip after it was pointed out that the original calcs were incorrect for tipped bullets. I linked his updated paper in that thread so it will be on here somewhere.

The Milspec stability minimum is 1.5, as accuracy tests for long-range military applications proved that values <1.5 aren’t acceptable. I’ve always selected bullets with stability values of >1.5 in my rifles as to my eyes they’ve always had better accuracy for what we do.

 

[ChesterP]

Great summary Nigel. It's worth also commenting that it takes very little runout of longer ballistic tipped bullets to affect stability markedly. I find Litz's stability charts in his Advanced Ballistics books a very useful reference although they don't cover all bullets, they cover many that I use. Happy to look up and share share this info for anyone needing a check done.

 

[MAG2022]

If you are brave enough for Facebook look up Bryan Litz Ballistics, or flush enough his books.

 

[MarinePMI]

If you are brave enough for Facebook look up Bryan Litz Ballistics, or flush enough his books.

Ding! Ding! Ding! We have a winner.

Material of the bullet really doesn't matter concerning stability. Center of gravity does. Bullets with the COG far forward stabilize much more easily than those with it set back. The further back, the less stable they are in flight. The further forward, and the ballistic efficiency suffers. And so the battle of compromise begins between flight efficiency and bullet stability...every bullet made is a compromise between stability and aerodynamic efficiency.

As Mag2022 stated, Litz has written (and spoken via podcast) on this very subject, and at great length.

 

[borbal]

Just to shake the tree a bit harder on stability, there are a number of questions I still have which are unanswered.

We all presume the stability calculators are right. After all some very clever mathematicians have written them. However, there are still anomalies and as I said the number is more a guide than gospel.

The calculators were I believe originally written for lead cored jacketed bullets - even artillery shells. With the advent of copper bullets with significantly lower density than lead does the calculation need to be tweaked a little to make the formula more accurate for copper or is density irrelevant? I know the formula doesn't have an input for density but should it - mathematically?

Also, the new lathe turned mono metal copper bullets are being built to tolerances of 2 microns. That means the central axis of rotation is exactly in the middle - well it might be 1 micron out. Compare this to lead core jacketed bullet with a variance of jacket thickness and a variance in now the lead settles within that jacket as well as the accuracy of the bullet dimensions, the central axis of rotation is likely to be many microns out. How does this difference in build accuracy effect the SF ratio number that's acceptable for good stability - perhaps good stability on a good lathe turned bullet is achieved at 1.2 rather than 1.3.

Lots to think about if you don't sleep much at night...

You are right. The stability factor does depend on density. And on my online stability calculator you can enter a density of your own. The density of copper/brass is about 8.4 grams/cc.

 

[ChesterP]

You are right. The stability factor does depend on density. And on my online stability calculator you can enter a density of your own. The density of copper/brass is about 8.4 grams/cc.

Very useful. Thank you for posting

 

[NigelM]

You are right. The stability factor does depend on density. And on my online stability calculator you can enter a density of your own. The density of copper/brass is about 8.4 grams/cc.

Thank you. I have been having a good play with your stability tool. Very interesting to see the difference you believe density has to stability.

The only downside I can see of your tool is that it does not account for tip length and material. Looking at how JBM believe the plastic tip length changes SF it seems to have a much greater effect than density.

 

[User00033]

Miller & Courtney:

Updated for plastic tips

 

[ChesterP]

A past, oft-quoted effect of some plastic tips on SF were found to be concerned with heat friction on external ballistics, specifically the melting of some plastic tips which affected both stability and variations in expansion. It was a difficult one to gather evidence for but the effects were real enough from tested precision at various ranges, more specifically from unexpected variations on precision which raised the issue. I can't remember which specific bullet manufacturer was highlighted but seem to remember it was a point of discussion a few years back.

 

[Dr. Strangelove]

A past, oft-quoted effect of some plastic tips on SF were found to be concerned with heat friction on external ballistics, specifically the melting of some plastic tips which affected both stability and variations in expansion. It was a difficult one to gather evidence for but the effects were real enough from tested precision at various ranges, more specifically from unexpected variations on precision which raised the issue. I can't remember which specific bullet manufacturer was highlighted but seem to remember it was a point of discussion a few years back.

I think it was Hornady with their AMax bullets. More specifically in 6.5mm but I can’t remember what cartridge. That’s what caused them to change from the AMax to the ELD-M & ELD-X, as I understand it.

 

[MarinePMI]

It was the Hornady's AMax and VMax bullets. And it was not so much related to a specific cartridge, but rather, a specific velocity (for extended periods of time). The 6.5CM just saw the greatest effect due to its wide community use and long range applications (where, as noted, the melting tips were causing erratic BC readings from high power pulse doppler radar chronograph readings at long range). It wasn't a lot, but enough to be measured, as well as have some effect on transonic stability calculations.