In the last post, we spent a lot of time talking about how to determine if is suitable in terms of game animal and hunting terrain. That discussion included a brief foray into a cartridges effective range which I defined as the maximum distance at which a projectile still carries sufficient impact energy for the target game animal. While that that’s important, it doesn’t tell the entire effective range story. In other words, just because one selected a .308 Winchester with a relatively heavy expanding projectile to hunt whitetail deer and the cartridge carries sufficient impact energy out to 800 yards doesn’t necessarily mean the rifle and the hunter will be effective for hunting whitetail deer out to 800 yards.
Why is that?
Let’s start this discussion by talking about the cartridge. For the sake of brevity, I’ll limit the discussion using Hornady’s .308 Winchester 178gr ELD-X Precision Hunter cartridge which has an advertised muzzle velocity of 2,600 fps. As advertised, this cartridge is effective for hunting deer and hogs to just beyond 850 yards as it carries more than 800 ft-lbs of energy to that distance. At 850 yards, this cartridge is going to drop about 246.1″ (assuming a sight height of 1.5″ and a zero distance of 100 yards). Now this is important, because the advertised muzzle velocity is an average. That means some shots will be a bit faster and some a bit slower. In my experience, this cartridge is fairly consistent which means the variance in muzzle velocity is relatively small. For the same of argument, let’s assume the standard deviation is plus or minus 10% which gives us an average velocity range of 2,574 to 2,626 fps. That translates to an average remaining energy range of 808 to 847 ft-lbs of energy and an average drop of 240.5″ to 251.9″ at 850 yards. Energy wise, the variance means cartridge is effective for this use to 850 yards. However, the variance in drop is 11.4″ which is 0.4″ to 1.4″ larger that the vital zone area of an average full grown whitetail deer. This implies that even assuming perfect shooting conditions and perfect shooting skill, there is a chance the impact at 850 yards will be outside of the vital zone.
Take note that I assumed a 10% variance from the advertised muzzle velocity of this cartridge. This is important for a for a couple of reasons which should become obvious as we continue this discussion. The important takeaway is that the cartridges effective range can be limited by trajectory variance. In other words, the cartridges effective range is also limited to the maximum distance where the trajectory variance is less than the size of the target which is the size of the vital zone of the game animal being hunted. The only way to be certain of what that distance based on trajectory variance it to use a chronograph to get some good velocity readings of the hunting cartridge and do a little bit of statistical analysis on it in order to determine what the actual average muzzle velocity and it’s standard deviation is.
This brings us to the rifle.
Chances are the barrel length on the hunting rifle will be different from the barrel length used to test the cartridge in order to publish advertised ballistics. For example, the advertised ballistics on this specific Hornady load were based on a test rifle with a 24″ barrel which is on the long side of barrel lengths for rifles chambered for .308 Winchester. Additionally, we don’t know if the rifle was a bolt-action or semi-automatic rifle. Let’s assume the hunting rifle is a semi-automatic with a 16″ barrel. This configuration will likely result in a lower average muzzle velocity. How much lower? It’s hard to say without actually taking some chronograph readings. For the sake of this discussion, let’s assume the average muzzle velocity is reduced to 2,400 fps. While still assuming a 10% variance, that yields a velocity range of 2,376 to 2,424 fps. That in turn translates to an average remaining energy range of 835 to 875 ft-lbs of energy and an average drop of 172.9″ to 181.1″ at 700 yards. This now means that this same cartridge which on the surface looked like a 850 yard whitetail deer cartridge is now a 700 yard cartridge when shot from a 16″ semi-automatic rifle. At least, from a ballistics point of view.
But wait, there is more.
Another limiting factor in the effective range equation is the precision of the rifle and cartridge combination which is usually limited even more by the shooter’s ability to shoot from various shooting positions. The best way I know to test this is to head out to the shooting range and shoot several multi-shot groups from various positions with the rifle and selected cartridge. To keep the math easy, I like to do this with a target placed 100 yards down range. After shooting a group, calculate the group size which is the distance to the outermost edge of the two most spread out holes minus the caliber of the projectile. For example, let’s say the distance to the outermost edge of the two most spread out shots is 2.308″. From that, subtract 0.308″ (the caliber of the projectile in a .308 Winchester cartridge) and we get a 2″ group. Now we can divide the target size by the group size and multiply by the distance. This determines the effective range of the shooter with that rifle and cartridge from that specific shooting position. Given a whitetail deer has a vital zone size of 10″ to 11″ (I’m going to use the smaller one as it’s more conservative), the math of this example is a 10″ target divided by a 2″ group times 100 yards yields 500 yards.
So what’s the actual effective distance? It’s the smaller of the effective ballistic range or the effective precision range for a given shooting position. Following the examples laid out in this post, the actual effective range is 500 yards.
One might be thinking that I’m suggesting that every hunter should go and purchase a chronograph and a bunch of ammo to do a lot of testing and math. One would be absolutely right if, and only if, the shooter’s effective precision range starts approaching (or exceeds) the cartridge’s effective ballistic range. At the very least, I think all hunters should take the time to determine their personal effective precision range with their rifle and selected cartridge. It’s imperative that we know the limitations of our ability and our equipment in order to avoid shooting beyond them.