Byron Pace takes a close look at bullet weight, sectional density, construction, velocity and how those features impact ballistic performance
To finish where we left off last time, we need to do a case study of sectional density to reinforce its importance. Below I have completed a test with two different weights of Sako .243 Win factory ammo. Both have the same construction, but one is a 90gr bullet and the other is a 100gr bullet, hence their sectional density is also different. Recall that the formula for SD:
Weight of bullet in pounds/diameter in inches2
90gr SD: 0.217
Full Penetration= 37.5cm, Secondary wound cavity= 17cm
100gr SD: 0.241
Full Penetration= 40.5cm, Secondary wound cavity= 21.5cm
We can change either the weight of the bullet or the calibre to alter the SD. Since our discussion touched on heavy for calibre bullets, the test here has been done with a .243 Win, altering the bullet weight while maintaining the type of bullet used. As a side note, I used to use the 100gr version of this in my own rifle, and it shot and performed tremendously well. In testing for this article, the 90gr ammunition showed superb accuracy from my Kimber Montana, and I will be making use of the rest of the box for some hunting this year.
For the test I adjusted the distance with which the ballistic gel was shot to counter the difference in muzzle velocity, so both bullets impacted at the same speed. Here the difference in SD is fairly small, and a larger difference would have been better, but it will still illustrate the point. If we were to have the same weight across two different calibres we would see similar results to the above, with the lower sectional density, and hence the larger diameter bullet, having the least penetration. What is interesting to note here is that the permanent wound channel in excess of the geometric channel is also greater with the heavier bullet – this should be expected with higher impact energy and mushroom potential of leading surface area. What will be obvious to some is that the diameter of the bullet has a much greater effect proportionally; in the formula this value is squared. A 10 per cent decrease in diameter from .243 in a 100gr bullet will increase the SD to 0.297. A 10 per cent increase in bullet weight increases the SD for the same .243 calibre only to 0.266. This is why heavy for calibre bullets offer the best sectional density.
For the same velocity, the permanent wound channel will be larger the bigger the leading area of the deformed bullet
It is, however, worth considering that we don’t always need high SD to efficiently kill an animal. This goes back to the idea of how much penetration we actually require. The high SD bullets so successful in the 6.5×55 and 7×57 built a reputation the world over, and were often used to kill game far bigger than would be reasonably expected from the calibre. This would suggest that for the six deer species in the UK, we can afford to be more relaxed with our sectional density requirements.
Take the 125gr bullet from a .308 Win. I can’t recall ever seeing this on a gunshop shelf, but it is loaded by Hornady, although not really intended for hunting applications. Even in the hands of re-loaders it rarely sees the light of day – however, it is favoured by our own Tim Pilbeam. In terms of SD it is poor, being short and fat in proportion. Ballistically however, it’s not all that bad, dropping by 5.2in with a 200-yard zero and a rise at 100 yards of just under 1in. Leaving the barrel at around 3,350fps, it delivers 2,240ft/lbs of energy at 200 yards. Compare that to the well-respected 150gr Power Shok from Federal, delivering 1,800ft/lbs and dropping over 8in, and you can see the appeal of the light .308 Win if it does the deed at the business end. Speaking to those who do use it, I am reliably informed it drops red deer with efficient incapacitation.
So we can see here that sectional density is not everything in all circumstances. The reason for this is that the momentum and penetration of the bigger, heavier bullets is simply not always required, and this light bullet still offers suitable penetration in certain game. It is true that it will not be as stable internally as the longer, heavier bullets, and nor will it cope with the frequent bush bashing seen in the thickets of Africa. However, assuming little likelihood of obstructions prior to reaching the target, this can actually be an advantage when it comes to quick kills in light-framed animals; tumbling and loosing stability once inside the vital organs. Remember that for the same velocity, the permanent wound channel will be larger the bigger the leading area of the deformed bullet. If it’s tumbling and twisting as well as expanding, this will be increased. In conclusion, high SD bullets are of serious consideration when shooting tough game such as elk, moose, large African antelope, ibex, and so on, but is of less importance in lighter-framed quarry. Furthermore, even high SD will not be enough if you are using a bullet of the wrong construction.
While touching on construction again, it is worth sharing two recent tests. The first used Hornady Superformance 75gr SST bullets, which by all accounts have been a massive success in the UK, ticking all the boxes for small deer and vermin in one feisty little cartridge. Carcase damage may be considered a little excessive in comparison to a more robustly constructed soft point bullet. This is primarily due to the bullet shedding a large portion of its weight (as we will see next time), but we are asking a lot of the ammo in terms of game spectrum. The key here is to take engine room shots a little further back, avoiding the front shoulder. With explosive performance occurring just on the other side of the ribs, roe don’t go far, with every broadside shot I have taken so far exiting on the opposite side. Leaving my rifle at just on 3,500fps, the super flat performance makes it so easily useable, with a drop of only 5in at 300 yards with a 0.9in 100-yard zero.
In my own load development I have been shooting Berger 69gr varmint bullets, which have become my go-to ammunition for foxing. A year or two ago I had some issues with the 87gr Hunting version of this bullet not providing satisfactory kills, which at the time I put down to the bullet simply being too hard and not providing suitable expansion. I will be looking at these again soon, with recent feedback from a fellow stalker suggesting they require a muzzle velocity in excess of 3,300fps in order to work – of course the question then is at what distance have they been tested. The important number is the impact velocity.
Despite my initial reservations of the Berger hunting bullets, there was no denying their supreme accuracy, prompting me to try the 69gr bullets I also had in stock. Shooting the first test load into a Clear Ballistics gel block at 100m, impacting just over 3,000fps, it was immediately clear to see that expansion wasn’t going to be an issue. They were doing exactly what they promised. Specified as an intended characteristic, the Berger travels almost an inch into the gel before showing any signs of expansion, thereafter expanding and fragmenting violently. Of course this means that the majority of the work being done on the bullet occurs inside the engine room – just where we want it. The SST shows similar performance but much greater weight retention and a less pronounced delay. We will return to this comparison next time, but it is curious to note that a cross section of the 75gr SST resembles a V-Max, whereas the heavier 95gr SST bullet sports the locking ring and cannular described in the bullets spec.
Having shot with both, it is hard to tell the difference in terms of terminal effect for turning the lights out. The Berger does have a tendency not to exit on roe, which many people won’t be surprised to find considering it has ‘varmint’ on the side of the box. My concern with this would be the lack of blood trail, however I have noted the entry hole is sizable and tends to leave sufficient blood for tracking, often breaking a few ribs going in owing to its shallow dump of energy. On foxes it is superb, and so far hasn’t let me down on either species. The key here is not to drive the bullet too fast. It would be easy to push it to speeds in excess of 3,500fps with N140 powder in a mindless drive for speed and flatter trajectory (I want my barrel to last). This would of course cause greater and quicker fragmentation. This will tie in with an upcoming investigation showing just how our bullets behave with varying velocities. It is possible that a varmint-designed bullet makes for a perfect deer hunting bullet at lower velocities. Pushing the Berger varmint just about 3,300fps seems to offer the best of all worlds. Certainly the internal effect of the bullet is devastating, as seen by what is left of the heart and lungs from a well-placed engine room shot on a roe. Next time we will dig into the design of these two bullets a little further.
Thanks to Defensible Ballistics for products and advice and GMK for supplying the Sako ammunition for testing.