Steve Hurt
In this age of rapid scientific advancement it’s sometimes difficult to keep abreast of the most current developments, even in our own areas of expertise and ballistic research technology is certainly no exception. Indeed, much of the research and development of recent years is tipping previous paradigms on their heads and challenging how we think about things altogether, long range bullet stability being a classic example.
I’ve previously written on how it’s possible to calculate a bullet’s likely stability at launch within the context of the bullet’s length, barrel twist and weight relationship – in a specific range of environmental conditions. The Miller formulae (there are three versions) will allow you to choose the correct bullet to get you on the money at launch but – and it’s a huge but – how will that bullet perform at extended range? Is it possible to calculate? The short answer is yes but it requires a level of information few shooters have access to so let me explain.
Before we start though, we must define the term ‘extended range’. In his books Modern Advancements in Long Range Shooting (Vol.1 and 2) Brian Litz defines long range as ‘when a bullet has to engage the transonic zone to reach the target’ – that is, between 1339 and 893 feet per second (fps) at the avionic standard of 15 degrees C and 1013mb of air pressure. This definition of long range is helpful but is limited as there are a number of additional criteria we may need to consider. Now if we’re only shooting at paper or modern electronic targets we could leave the Litz definition right there and it would be perfectly acceptable. Hunters, however, have a problem.
The challenge every hunter faces is there are very, very few bullets which will expand at velocities below 1600fps and, depending on the cartridge driving it and the intended target, also deliver the energy required for terminal effectiveness on medium-sized game. Specialty bullets which expand at subsonic velocities can’t generally be relied on supersonically without risk of failure at short range, or even in the barrel of modern centrefire cartridge rifles.
The limiting factors of expansion velocity and sufficient energy delivery pull hunters up a long way short of transonic engagement, so extended range application for an ethical hunter is limited to the 1600fps or faster benchmark, where most bullets which were stable at launch are likely to remain so.
Hunting bullet manufacturers consider accuracy and terminal performance as their major objectives, not transonic performance as this product group wasn’t designed to punch holes in paper at ‘long range’. Once a hunting bullet’s velocity falls below its designed expansion velocity/range, performance of any description beyond that limit is no longer relevant. Besides this there are design conflicts for the bullet manufacturer. The attributes which enhance terminal performance are often in conflict with transonic target accuracy but that’s for another time.
Determining long range stability
There are two ways to determine the suitability of a particular bullet for long range application, just as there are for load development. The first is trial and error and we’re all familiar with that one. The second is the scientific approach as the primary value of science is to reliably predict outcomes. When theoretical mathematics and observed outcomes are the same, it’s reasonable to assume the science is sound. If they don’t agree there’s either something wrong with the theory, the maths or perhaps the advertised claims but not the outcome.
For a bullet to survive the transonic zone it must have certain attributes and each one has to be perfectly balanced within the total package. The Miller formulae are a great start in determining a bullet’s stability at launch but these simple formulae don’t accurately reflect the dynamic changes in behaviour of a bullet in flight. A more complex application of the maths is required, especially at the inversion of pressures applied to a bullet around the sound barrier. Test pilots in the early days of trying to break the sound barrier found this out the hard way and, sadly for some, at the cost of their lives.
From our understanding of the Miller formulae we know a bullet should be launched with a stability factor of at least 1.5, but at no point throughout the entire flight path should a bullet’s stability fall below 1.0 (where the bullet will tumble) and the Miller numbers don’t accurately reflect results at the transonic intersection.
For this we need a more sophisticated program and a layman’s version can be found on the JBM website. The problem is these calculations are only as good as the data input and unless the shooter knows the exact dimensions of a bullet’s meplat, compound ogive radius and length, total length and boat tail proportions, accurately expressed in calibres, the shooter is none the wiser.
So where to from here? The takeaway message is that in reality, long range bullets designed to engage the transonic zone are most likely to be target bullets rather than hunting bullets – and even then they’re a specialty subset within the target group. If this is your game then it’s imperative you follow the barrel twist recommendations of the bullet manufacturer and check the actual specifications of your barrel. It’s often a shock and disappointment to many customers to find their 10” twist barrel as specified turned out to be a 10.5” twist when measured and yes, the balance can be that fine, meaning the difference between success and disappointment. If you find yourself in this unfortunate position there’s little choice but to use a slightly shorter bullet or change your barrel for a faster twist edition.
There’s always more to learn and I don’t make any claim as to having the whole picture so if there are any questions or comments which might advance the science of this conversation (pointing to the evidence), I’d love to hear them.