Leaving sectional density behind us, it is probably wise to add a brief discussion on the ballistic coefficient (BC) of a bullet for those who are not familiar with it. The ballistic coefficient in itself will have very little effect on terminal performance, so for our discussion I have included it mainly for completeness. It is a specification often bandied about, but normally with little elaboration, and in truth, for average hunting distances the BC of a bullet is of little concern.
The BC, in simple terms, is how aerodynamic a bullet is. The number referring to the ballistics coefficient is measured in lb/in2, and can be derived either using a G1 or G7 form factor (G7 being closer to modern long-range bullet designs). At its most simplistic level, these refer to a standardised bullet shape, with the ballistic coefficient being a measure of how closely a bullet design matches this, taking the sectional density into calculation to provide the final numeric output.
Two bullets of the same weight but different design will have the same sectional density but could have very different ballistic coefficients. This is important for long-distance shots because a high BC means greater velocity retention and less wind deflection. Over practical hunting distances it really makes little difference. In fact, for bushveld conditions, blunt-nose bullets perform better, being less likely to deflect if encountering foliage. Blunt bullets also hit the target harder, already having a leading frontal area close to the bullet diameter – unlike a pointed bullet, which must first expand to achieve this. These will of course lose their velocity much quicker than pointier bullets, but that is of little concern over shorter distances. We only really need to be concerned about BC for shots in excess of 300 yards, which for average hunting is a rare thing indeed.
Just to demonstrate the point, included here is a table of four .308 Win bullets of different BCs and their respective bullet drop. As you can see, despite a marked difference in BC the difference in drop over reasonable hunting distances is negligible.
We are now getting to the point where ammunition manufacturers and their resulting bullet designs will find themselves pitted head to head. You will see the first of these in a forthcoming issue, when I unpack the question as to whether the A-Max bullet from Hornady really is suitable as a hunting bullet, comparing it to some industry benchmarks. For now, we have a couple of concepts to conclude, building on the base we have already established for understanding a bullet’s terminal performance.
Back in the fourth article in this series we saw the marked difference between the terminal effect of a .22-250 Rem bullet traveling through bone when comparing fragmenting and bonded bullet designs. As was to be expected, a thicker-jacketed bullet, or indeed a solid monolithic bullet, will cope with the stresses of dense bone, with less impact on the bullet than a thin-jacketed varmint bullet designed for immediate, rapid expansion. Although this may be obvious, the premise is important, as bone and muscle are substantial barriers that have to be overcome before we can deliver fatal damage to the vital organs.
Having successful penetration of the ‘barrier zone’, we should be far more concerned about the bullet’s performance in lungs and heart than with muscle. We need our bullet to successfully cope with this passage, and be in a suitable form and state for maximum killing efficiency as it passes through the engine room. The ‘barrier zone’ on some animals is thicker and stronger than others, and calls for a different bullet and/or calibre.
It is easy to see why you would want a strongly constructed bullet when shooting through the shoulder of a moose, and why a fragmenting bullet would be a poor choice. As the test between the Fusion and Nosler B-Tip bullet showed, the state in which each of these bullets would reach the killing zone is very different, if indeed a light fragmenting bullet made it through the vitals at all.
Taking into consideration thick-skinned animals, or those carrying dense coats, with strong heavy frames, there is no doubt that our bullet has to undergo considerable stresses before it reaches the vital zone. As always we have to choose a compromise, as for ultimate bullet strength and penetration, a solid would be the answer (we will cover these for dangerous game at a later date). The non-existent expansion seen with such bullets may be excellent for smashing bones, but provides very poor terminal effect on internal organs.
Bullet designers have battled with the trade-off of penetration and expansion, leading to the impressive and consistently performing bullet designs we have available today. A lot of hunters don’t give it a second thought, simply shooting ammo that a manufacturer recommends as suitable for the quarry they are hunting. Making the correct choice really should weigh more heavily on our decision-making process. I am confident that the results I will present over the rest of this year will show that we tend to use bullets that are not only too heavy, but also too robustly constructed for delivering the most efficient kills possible.
We really will be looking at results on the margin, as the reality of hunting the smaller, light-framed game that we mostly have in the UK is that just about any expanding/fragmenting bullet placed in the engine room will deliver fatal internal damage over moderate ranges. Some combinations of calibre, bullet weight, construction and velocity are obviously better, and we will see the result of this. Looking on further, the most important result from the search into suitable bullet/cartridge combinations will be for bigger, tougher game, especially when using magnum cartridges.
As an initial test I set up a comparison to look at how different bullet constructions coped with extreme stress. This would not only give an indication of how well a design could cope with shooting through tough skin, dense muscle and bone, but would also give an indication of how well a bullet would cope with higher-velocity impacts.
This test couldn’t be much simpler, consisting only of a box of compressed newspaper and magazines. This is a poor representation of real-world conditions, and the results in terms of penetration are not particularly useful. It only serves to stress the bullets in a way that will still allow them to undergo penetration and deformation. It is akin to shooting through bone.
This test was completed with three bullets from Geco. The Express, Teilmantel and Plus bullets offer a good spectrum of design types in the same .308 bullet.
Most satisfyingly, the results played out just as I excpected. You can see this in all its glory on The Shooting Show. The standard soft point and polymer-tipped bullets shed most of their weight, leaving only the copper jacket at their furthest penetration. The lead was strung out along the wound path. In contrast, the bonded Geco Plus held up tremendously well, retaining almost 80 per cent of its weight, penetrating considerably further. Not only would the wound channel have been more evenly distributed along its journey, retaining its form well, it would also have suffered the least from deflection away from the vital zone.
At this extreme, the bonded bullet clearly shows its advantage – that said, they are not a magic solution. Quicker kills are more likely in medium-weight game shot with non-magnum calibres using standard cup and core-type bullets.