FEATURE Steve Hurt
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Frank Zappa, a rocker from the 70s to the 90s, coined the term ‘Factoid’. His definition of a factoid was “an idea discussed often enough, long enough, by enough people, to be considered truth, even when there is not a shred of evidence to support it.”
At Outer Edge Projectiles (manufacturers of copper bullets) we hear a lot of this kind of banter. New technology is like that…
This article is not a ‘sales’ article intended to persuade anyone of the ‘rights or wrongs’ of copper vs lead bullets, but rather an explanation of the basic science of the differences between them, and what this might mean for the hunter in application. I would like to thank the ADA for the opportunity to explore this subject from an albeit limited space perspective. So, let’s clear the air regarding a number of commonly held ‘factoids;’ not just for copper bullets, but bullets generally.
Cutting to the chase…
Petrol vs Diesel
The best way to look at this issue from the author’s view, is to consider the comparison between petrol and diesel.
Which one is best? Well, that depends on lots of things, and the answer isn’t always as simple as we would like it to be; but I think we can all agree that whichever you choose, it is important you don’t mix them up!
If you put petrol in your diesel engine, or diesel in your petrol engine, you are going to have problems. It’s the same with copper (Cu) bullets vs jacketed lead (J/L). They work differently and shooters need to understand the differences if they are to get the best outcome.
Jacketed lead (J/L) bullets – how they work from a terminal perspective
From a hunter’s perspective (we will ignore target and military products), J/L bullets come in two distinct categories: core bonded and basic swaged.
Testing consistently demonstrates that any J/L bullet that is basic swaged and not core bonded, when impacting a medium or large game animal like a large deer, will lose somewhere between 75 and 90% of its mass on impact. This produces a massive energy dump into the animal but is severely limited in its capacity to penetrate deeply enough to reach the vitals, except perhaps on a perfect broadside or head/neck shot.
Core bonded J/L bullets again fall into a number of categories; mechanically crimped, chemically bonded (think soldered), partitioned or a combination of these. Weight retention varies enormously, as do other terminal impact qualities. Some mechanically crimped bullets will easily lose 50% of their mass on impact, while a chemically bonded bullet may only lose 10 to 25% of its mass on impact. Partitioned bullets usually fall somewhere in between. Penetration is generally deeper, with terminal performance considered more reliable, giving flexibility in shot angle options to still reach and destroy the vitals for a humane outcome.
Copper bullets (Cu) – how they work from a terminal perspective
Like J/L bullets, Cu bullets also vary enormously in manufacturing and terminal performance. In the early days, Cu bullets were made the same way as J/L bullets. They were swaged into shape; had a petal mandrel punch a hollow point and relieving grooves later cut to reduce pressures. These weren’t particularly aerodynamic, and because of the swaging process, work hardened. The result was a product that worked, but was limited in application. If they weren’t driven hard, they wouldn’t expand. If they were driven too hard, the petals would break off and the shank would pin hole. If they weren’t driven hard enough, you ended up with the same result; poor to no expansion, with pin holing, leading to unsatisfactory terminal outcomes. Effective performance range was specific to the cartridge, generally limited and misunderstood.
That was 60 years ago. The world has moved on since then.
Although some Cu bullets are still made this way, many more modern Cu bullets are likely to be lathe turned on Swiss Sliding CNC machines. This gives manufacturers far greater control over material hardness and ductility, because the chosen raw material leaves as finished product, at close to the same temper as when it was input. Tolerances are also unmatched by any other production process.
A properly designed and manufactured modern Cu bullet differs from a J/L and older style Cu bullets in that 1.6 – 2 times mushroom expansion should be considered routine within the manufacturer’s nominated operating range, with 95% or more weight retention. Bullets constructed in this manner have a hydrostatic shock wave that is the square of the newly expanded meplat (bullet tip), while at the same time maintaining mass for optimal momentum and therefore penetration. This gives the hunter greater flexibility in reaching the vitals from a less-than-ideal angle, as this style of bullet consistently cuts the straightest path through flesh and bone of any hunting bullets made, flat-nosed solids aside.
So, what does this mean for shot placement?
Hunters using J/L bullets tend to aim behind the shoulder, as historically shoulder shots varied bullet performance to such an extent as to make the terminal outcome unpredictable. Shooting behind the shoulder has less structural interference with the possibility of striking the back third of the lungs, on a perfect broadside. The challenge is that the heart and most of the lungs of the deer are right between the shoulders, and this is exactly what we are generally trying to destroy with perfect shot placement. Copper bullets that expand without losing their mass will smash the shoulders, routinely destroying the heart and lungs from a much greater range of shot angle options. In short, if you are using Cu bullets, don’t be afraid to shoot them in the shoulder! Any animal shot through the shoulder/s, heart and lungs won’t be going anywhere.
Bullet weight and the question of sectional density (SD)
Back in the 1890s through to the 1950s, bullets were all made essentially the same way. Yes, there were a few novel designs being experimented with, but they weren’t common. In that world, bullets were compared by weight and sectional density. Although this wasn’t an exact science, it was good enough to make some basic comparisons in bullet terminal performance.
A bullet’s sectional density is defined as the weight of the bullet in pounds (converted to grains), divided by the square of its diameter in inches. This goes hand in hand with the principle of momentum.
The fundamental flaw in this comparison is that these calculations are done in the static form, that is, when the bullet is in the magazine doing nothing. The real question is, what is the weight of the bullet and its diameter when it is actually doing the job? For instance, a 30-calibre J/L bullet that weighs 200 grains has an SD of 0.301, sitting in the magazine. If its expansion is doubled, its sectional density is now 0.150, provided it maintains full weight retention, which in a J/L bullet is most unlikely. If that same bullet loses only half its weight, its SD is now 0.075! Compare this to a 30-calibre Cu bullet that only weighs 150 grains, with a static SD of 0.226 sitting in the magazine. Assuming realistic double expansion, the SD of this bullet is now 0.113 during terminal performance. Again, assuming that the Cu bullet only loses 5% of its mass on impact, which is very realistic, this bullet’s SD is now 0.107: or more than 40% greater while it is actually doing the job.
Yes, that’s correct, the lighter Cu bullet is both heavier (150 grain x 0.95 = 142.5 grain vs 200 grain x 0.50 = 100 grain) and greater in SD while it is doing the job!
So, what are the implications for bullet selection?
The accusation is often made that copper bullets punch straight through, without delivering the energy. This is usually a function of ill-informed bullet or cartridge selection, based on old style product, or old style thinking in relation to initial bullet weight at launch.
There is such a thing as too much and not enough of anything; and when it comes to momentum this is certainly the case. Within reason, the author suggests that when considering Cu bullets, going down in bullet weight as the better option to reduce the likelihood of over penetration and maximising energy dump.
For example, if you are used to using a J/L 30-cal bullet in the 180 – 200 grain class for hunting sambar, a better matched Cu bullet selection would be in the 140 – 160 grain range. The lighter copper bullets will fly faster and flatter within normal hunting ranges, and with less recoil!
There are two distinct differences between J/L bullets and Cu bullets that must be kept in mind when loading for optimal performance!
1. Matching your bullet to your barrel twist
All bullets, regardless of type, require a certain level of stability to be consistent, accurate and terminally reliable. This is a function of bullet length to barrel twist to bullet weight ratio, best described as the Miller Formula for those who wish to explore the matter further.
Delving into this is usually beyond the scope of the average hunter, who just wants to pick up his or her gear and know that it will do the job required.
That said, in the same way that Cu bullets differ to J/L bullets in terms of weight comparison for momentum and sectional density, they also differ in terms of the bullets being matched to the rifle barrel twist rate, and this is a very important issue.
Commercial rifles (and their barrels) are generally limited in their twist rate options. This will normally require a shooter to choose a lighter Cu bullet when compared to a J/L product, because, as Cu is lighter than lead, it is also proportionally longer. As a general rule, if two bullets were of exactly the same external dimension, with exactly the same displacement, but one is made of Cu and the other J/L, the J/L product will be approximately 20% heavier.
This means that if your rifle stabilises a 30-cal 180 grain J/L bullet, it will likely run a 150 grain Cu bullet and still be suitably stable. While this is only a rule of thumb, it’s close enough for the vast majority of hunting applications.
Quality bullet and ammunition manufacturers will state exactly what barrel twist rate their particular product requires for stability. All any hunter really needs to do is understand this and follow the manufacturer’s recommendation for best results. It’s as simple as that.
2. Jump to the lands
The next major issue for hand loaders is ‘jump to the lands’
Most handloaders used to loading J/L bullets will be in the habit of setting the bullet jump to the lands at somewhere between 0.007” and 0.020.” Whilst this may work for many J/L products, it definitely won’t work with Cu bullets.
Copper bullets, when hand loaded, require a minimum jump of 0.050,” with 0.060” being more common. This is a function of material ductility and Shot Start Initiation Pressure (SSIP).
Most J/L bullets come within a relatively small range of SSIP, usually coming in on average at around 3,626 psi, but not so Cu bullets.
Old style Cu bullets had an SSIP of between 4,500 and 6,000 psi, which is considerably higher than J/L product, while modern Cu bullets come in much closer to the original 3,626 psi mark.
While modern Cu bullet designs are closer to conventional J/L products at around 3,626 psi, they still require the longer jump regimen of 0.050” – 0.060”.
Also essential is that, regardless of the product chosen, it is imperative that you follow the manufacturer’s load guidelines! Bullet weights from one brand to another definitely do not translate in load data!
In our next article, we will explore the principles of energy, how it is delivered, and how to calculate the practical limitations of any chosen cartridge/bullet combination.
In other words, ‘how far can I expect my chosen cartridge to work reliably?’
“The challenge is that the heart and most of the lungs of the deer are right between the shoulders, and this is exactly what we are generally trying to destroy with perfect shot placement.“
Australia Deer magazine Editor