Range Report Stability Formula for Aluminum Tipped Bullets (Spreadsheet linked)

[Michael Courtney]

Most know that Don Miller published an accurate formula for computing gyroscopic stability of constant (or near constant) density bullets in 2005. This formula has been incorporated in several ballistics calculators including JBM, ColdBore, etc. Don knew that his formula would be overly conservative (predict low stabilities) for bullets whose density varied significantly along their length, so when I contacted him in 2010, he was eager to collaborate and develop an accurate formula for plastic tipped bullets which we experimentally validated in 2011 and published in Precision Shooting in early 2012. This formula has also been incorporated in several ballistics calculators.

Before Don passed away in 2012, he expressed a desire that his formula be adapted for open tipped match bullets and we shared some ideas for the development and testing of a stability formula for open tipped match bullets. This formula was validated experimentally and published earlier this year.

As aluminum tipped bullets have become more popular, the requests for info on stability of aluminum tipped bullets have increased, so yesterday, we finally added a formula for aluminum tipped bullets to the spreadsheet. Due to the paucity of aluminum tipped bullets, this formula has not yet been experimentally validated, so for now, we are estimating its uncertainty as 10% rather than 5% for the constant density, plastic tipped, and open tipped match formulas that have been experimentally tested. This formula is essentially a linearly interpolation (or weighted average) between two formulas that are known good (the constant density case and the plastic tipped case).

Like all ballistic calculators, the accuracy of the outputs depends on the accuracy of the inputs. You really need an accurate bullet weight, total length, length of the full density portion, twist rate, and environmental conditions. A reloading scale, calipers, and Kestrel are sufficient, but the claimed barrel twist rate of the manufacturer usually is not. There are some good sources on measuring it yourself.

The spreadsheet is linked below. We welcome valuable experiential feedback on the accuracy of our formulas, but feedback is difficult to assess if the inputs have not been measured with confidence and if it does not include specific observations about why you think the bullet is or is not stable. Theoretical discussions are less valuable unless you are comparing our predictions with those of PRODAS. Accuracy observations are harder to relate to stability than observations of key holing, significant yaw, or accurately measured ballistic coefficients.

The adapted stability formula generally predicts a higher gyroscopic stability for aluminum tipped bullets than the original formula that assumes constant density. This is because the moment of inertia about the tumbling axis is lower for aluminum tipped bullets than for bullets of more constant density. The practical result is that a given rifle barrel might actually stabilize an aluminum tipped bullet in cases where stability of a constant density bullet of the same weight, caliber, and length, might me marginal or less than 1.0.

Stability Formula for Aluminum Tipped Bullets (Spreadsheet attached)

 

[Emouse]

Mike,.I take it you mean the ULD profile secant bullets? These are still the territory of a select few custom bullet makers and then nearly all on the Corbin dies that have been around since late Vietnam era? Those that shoot them keep the data close to their chest,..with good reason. Its easier today to accurately determine specific barrel twist rates given the production of CNC manufactured barrels. Maybe a chat with Rocky Mountain Bullets will give you the data confirmation you need? He makes a tipped .338 ULD that flies damn fine.

 

[Michael Courtney]

Mike,.I take it you mean the ULD profile secant bullets? These are still the territory of a select few custom bullet makers and then nearly all on the Corbin dies that have been around since late Vietnam era? Those that shoot them keep the data close to their chest,..with good reason. Its easier today to accurately determine specific barrel twist rates given the production of CNC manufactured barrels. Maybe a chat with Rocky Mountain Bullets will give you the data confirmation you need? He makes a tipped .338 ULD that flies damn fine.

Thanks for your ideas and insights.

We're aware of the bullets with the Corbin dies, but a number of parties have taken to adding the Corbin aluminum tips to various commercial bullets by trimming the tips and prepping the tips to accept the aluminum tips.

Not everyone who has contacted us about aluminum tipped bullets has given all the details. We gently hint that we'd like to know all the details of the bullets they are using, but at the end of the day, all we really need to know are the total length, the Al tip length, the weight, the caliber, the twist rate, and the environmentals.

We've had varying success getting dimensions from bullet makers. Bullet makers are also coy about how conservative their twist rate recommendations really are. The data needed to test the formulas isn't really the overly conservative twists rates that work with a big margin for error, but rather the twist rates that barely work or that fail to work and produce tumbling.

I think Al tipped bullets will become increasingly important in long range shooting.

 

[Emouse]

Mike,..the Corbin dies assume you are making a 14 secant bullet. You may have a hard time fitting that to anything else. Hornady did make an ally tipped AMAX (STILL DO IN .50) and it was far the best bullet they have fielded but they didn't continue due to tooling issues long term. BC isn't everything and these bullets do require careful load development in terms of seating depth and also some thought on lead angles, however straight throats do help along with faster twist rates. Current designs run less secant and almost a hybrid design so that shorter bullets may mag feed. When you get it right they excel and outperform any retail commercial bullet.

Aluminum or plastic tip,..matters not as long as the basic design is good. Secant and tangent ogives can be run almost pointed and as long as uniform dont need "tips" to be super accurate.

I am not sure a specfic stabillity formula for anything this fringe is actually worth the effort as those that shoot such bullets simply "get out and test",..for me when I see significant MOA knocked off windage and elevation compared to a benchmark commercial bulet its both interesting and definitely benificial.

 

[Michael Courtney]

Mike,..the Corbin dies assume you are making a 14 secant bullet. You may have a hard time fitting that to anything else. Hornady did make an ally tipped AMAX (STILL DO IN .50) and it was far the best bullet they have fielded but they didn't continue due to tooling issues long term. BC isn't everything and these bullets do require careful load development in terms of seating depth and also some thought on lead angles, however straight throats do help along with faster twist rates. Current designs run less secant and almost a hybrid design so that shorter bullets may mag feed. When you get it right they excel and outperform any retail commercial bullet.

Aluminum or plastic tip,..matters not as long as the basic design is good. Secant and tangent ogives can be run almost pointed and as long as uniform dont need "tips" to be super accurate.

I am not sure a specfic stabillity formula for anything this fringe is actually worth the effort as those that shoot such bullets simply "get out and test",..for me when I see significant MOA knocked off windage and elevation compared to a benchmark commercial bulet its both interesting and definitely benificial.

Thanks for your comments and your insights. Given the number of inquiries we had about aluminum tipped bullets and the fact that we were distributing the spreadsheet privately, we decided to make it public so as not to give any (real or imagined) advantage to those to whom we had sent it.

I'm sure there are many parties who just "get out and test" but many with whom we had corresponded were either putting together a rifle for a specific bullet, or trying to optimize a bullet weight and length (and BC) for a specific twist they already had. F-Class and other long range shooting is an expensive game, and many shooters like to increase their confidence that an idea is more likely to work before spending money on a custom barrel or a batch of custom bullets. I think the spreadsheet has been downloaded 50+ times in the first two days, so the stability question must be rattling around in a few shooters' heads.