Ammunition Transition Case Study
MARK'S STORY
"As a deer manager I have used non-lead ammunition for the past four years and have found it to be both accurate and effective when culling deer.
That said, I used a .308 and before transitioning I read as much as possible to ensure that I would quickly find a suitable round for my rifle.
In the process I realised that it is not just a simple case of switching ammunition, one needs to understand the internal ballistics of the gun you own…
Key to transition success is understanding the twist rate in your barrel. Traditionally, manufacturers have used a twist rate that has been appropriate for the intended cartridge, and until recently this has been for a lead core bullet.
The twist rate is critical because it imparts gyroscopic stability, which is what keeps the bullet flying point forward.
For many years the relative proportions of bullets have not changed, and as such twist rates have remained around standard values. With the impending introduction of non-lead ammunition, bullets have become longer and as such are too long to fully stabilise in conventional twist rates, particularly in smaller calibres.
Using a tool such as the Berger Twist Rate Stability Calculator, you can predict the likely stability of any given bullet in your rifle, as long as you know the twist rate of your barrel.
A gyroscopic stability factor (SG) between 1.0 and 1.5 indicates a marginally stabilised bullet which means that it will fly with some amount of pitching and yaw (rotation of the nose of the bullet away from the line of flight). This will induce extra drag and will reduce the bullet’s effectiveness.
At short ranges this may not be too much of an issue, but it does affect accuracy and ambient temperature, and pressure can further exasperate this error. An SG below 1 indicates an unstable bullet, which may start tumbling as soon as it leaves the barrel.
As a non-scientific trial at home I used the twist rate stability calculator to look at one bullet that has recently been released to market in the UK at .243. As part of my trial, I also used three different twist rates of 1:9, 1:10 and 1:12.
When using a twist rate of 1:9, the calculator predicted a stable bullet. A twist rate of 1:10 was only marginally stable at an ambient temperature of 20 degrees centigrade and produced an SG of 1.28.
When I then reduced the ambient temperature to zero degrees the SG reduced to 1.19 indicating increased pitching and yawing. A barrel with a twist rate of 1:12 could only produce an unstable bullet in flight.
Increasing the muzzle velocity (MV) can improve stability of a marginally stable round, but runs a significant risk of making the rifle dangerous by exceeding its proof.
Statistics suggest that .243 calibre rifles account for almost 70% of all rifles used for deer management in the UK.
While gun manufacturers are developing rifles with better twist rates that are more suited to firing non-lead ammunition, many are still selling rifles with twist rates of 1:10 which appears to be marginal at .243 calibre.
It is also likely that older rifles (more than five years old) may even have twist rates of 1:11 – 1:12, which will make it extremely difficult to find a suitable bullet for deer management activities.
What is clear, is that users must know the internal ballistics of their rifle before switching to a non-lead bullet. Information on the twist rate for your rifle might be found online.
For older rifles, you may need to assess this for yourself using a cleaning rod and counting the number of turns over distance. There are a number of videos explaining this on YouTube, and while this is not scientific it will provide a pretty good indication.
Finally, when transitioning to non-lead ammunition you must ensure that the ammunition being purchased is suitable for the twist rate for your rifle and is legal for its intended purposes."
- Mark Lacey is an active deer stalker and has helped the Woodland trust as well as continuing to assist DDM in Wales. With a background in Defence Procurement he has been intimately involved in weapon design, as well as safety.
