Selecting a Rifle Cartridge

Introduction

In my last article, Selecting A Rifle: An Organized Approach, I discussed an approach to rifle selection. If you’re a soldier, it’s pretty easy – the Army presses a rifle into your hands. The approach I describe is more general, based on a number of different applications. Varmint hunting, midsize game, large game, target shooting, long range shooting, police marksmanship – any number of mission sets.

To follow this approach, you need a good understanding of your target and your tools. The latter means an understanding of cartridges in general, bullets, and ballistics. Armed with that understanding, you can decide what bullet and cartridge are best suited for your target. That decision will guide you to select the right rifle for your application.

With that in mind, this article will give you an overview of cartridges and the issues involved in selecting them. The process doesn’t stop there. As you’ll see, this material simply informs your selection of bullet, that which delivers the smack to put your target down.

The Cartridge

Figure 2 shows a cartridge that will be fired by your rifle (or pistol). It consists of two parts. The first is a shell casing that contains a charge of gunpowder. The second is a projectile – the bullet. When you fire your rifle, the powder in the shell casing ignites. The burning of that powder causes gasses to expand and propel the bullet down the barrel. Grooves in the barrel – the rifling – spin the bullet to keep it stable in flight. The barrel flies in what is called a ballistic trajectory to your target.

Now you can go down to the store a buy a box of cartridges like the ones shown in Figure 3. These are Hornady 195-grain ELD (Extra Low Drag) Match-grade .300 Winchester Magnums.

The external ballistics of .300 Win Mag match bullets make them very good flyers at ranges over 1,000 yards. In fact, the military switched to .300 Win Mag for snipers when they found the external ballistics of their standard .308/7.62mm NATO cartridges were wanting at ranges in excess of 1,000-1,100 yards. They switched again when they wanted to reach 2,000 yards. More on that below.

These bullets are designed to remain stable and hold their velocity longer. They’re constructed like hollow points. Hollow point ammunition in handguns is designed to expand quickly for stopping power. Hollow point and polymer-tipped match and target ammo are different – it is meant to shift weight out of the nose to the back of the bullet. That shifts a bullet’s center of gravity and aids stability. We’ll talk more about that in a future article.

Now, you can buy cartridges off the shelf, or you can load your own. That means you buy the shell casings, the primers, the powder, and the bullets separately. Then you make your own cartridges. This gives you more choice, and it allows you to fine-tune your weapons system in a number of ways. In this series, I will not discuss hand-loading.

Why Ballistic Coefficient Matters

What do we care about when we select a cartridge? At least, in the first pass, before fine-tuning? What we want is a table like the one shown in Figure 4. It lists a set of cartridges with various metrics of interest. You’ll find our .300 Win Mag is sixth from the top.

We can look at the table and decide which cartridge suits our needs best. Suppose we want to operate at ranges of 1,000 to 1,200 yards. For the round we’re considering, the .300 Win Mag, Hornady ELD Match, we can list the metrics we’re interested in. Then we compare those metrics to those of other cartridges we think will meet our requirements.

One of the most important metrics is the Ballistic Coefficient. This number captures the ability of the bullet to remain stable and hold its velocity over longer ranges. If you are shooting at short ranges, BC is not that important. At longer ranges, it becomes more important. If you are shooting at 800 yards or more, BC is something you want to look at carefully.

The .223 Remington, of 55 grains, is a popular round for the AR-15 platform. It’s not in the table, but I can tell you that, depending on the manufacturer, it has a BC of around 0.200. The 67-grain Sierra OTM (Open Tip Match – effectively a hollow point) has a BC of .270 and could get you to 600 yards. The .223 was meant for assault weapons and combat under 200 yards. By 600 yards, its performance will deteriorate.

The .300 Win Mag 195-grain ELD Match has a BC of .584 and, no surprise, is far better at long range than the .223. The .223 loses its oomph at 600 yards, but the .300 Win Mag is good to 1,500. The bullet is still supersonic at 1,500 yards.

Compare that to an otherwise similar cartridge, the .300 PRC (Precision Rifle Cartridge). The PRC was designed by Hornady to US Special Operations Command specifications. For special operations, the DOD wanted increased first-round hits on targets at extreme ranges up to 2,000 yards. Barrett Firearms was awarded a contract to produce an unspecified number of MRAD Sniper Rifle Systems in .300 PRC (see Figure 1)

The .300 PRC 225-grain ELD Match has a .777 BC. Now consider that while its muzzle velocity is lower than that of the .300 Win Mag, its retained velocity at 1,500 yards is actually higher. In fact, the .300 Win Mag velocity falls behind quickly. That’s not all – the .300 PRC requires less elevation at 1,500 yards to compensate for bullet drop (expressed in MOA in the Hornady ballistic calculator they used to prepare this table). The PRC is also subject to less wind deflection.

The .300 PRC 250-grain A-Tip has a BC of .878. It is worth noting that not all manufacturers offer rifles chambered in .300 PRC. This is a case in point where your choice of cartridge can affect your choice of rifle. Furthermore, it’s well worth considering the availability of your cartridge choice. You will always be able to find .223 Remington and .308 Winchester.

The AR-15 is a very accurate weapon. If you are going to be working well inside 400 yards, it’s fine. At longer ranges, you can’t just look at a rifle, a cartridge, and its muzzle velocity. You need to consider its trajectory over your entire working range. You need to consider how much velocity it will retain, and even more important, how much energy it will retain when it hits your target.

Depending on why you are considering an AR-15, you might want to consider an AR-10 chambered in .308 Winchester. The .308 Winchester has a BC of .505 (Sierra Match King). Other .308 rounds have BCs in the low .400s, so be careful. The AR-10 is also a semi-automatic platform and the .308 is a more capable round. It is, however, heavier to carry, both rifle and ammo.

Conclusion

In general, if you are considering applications over longer ranges, you want cartridges with a BC of at least .400. You’ll find lots of choice, even in the .243 and 6.5mm cartridges. Cartridge choice does matter. We have also seen that the performance of a cartridge is highly dependent on its projectile. For example, the .300 PRC 225-grain ELD has a BC of .777 and the 250-grain A-Tip has a BC of .878 The two bullets will exhibit differences in trajectory and performance.

The purpose of this article was to frame the issues around cartridge choice. I’m neither pushing nor knocking any of the cartridges discussed here. I’ve used them as examples to show you the kind of analysis one can do. In the next article, we’ll look more closely at Ballistic Coefficient. That discussion will also introduce concepts of center of gravity, center of pressure, and bullet stability. We will find that the BC, and modifications thereof, is important in predicting the trajectory of a bullet’s flight.

About the Author

You may reach Cameron at: cameron.curtis545@gmail.com

Cameron Curtis has spent thirty years in the financial markets as a trader and risk manager. He was on the trade floor when Saddam’s tanks rolled into Kuwait, when the air wars opened over Baghdad and Belgrade, and when the financial crisis swallowed the world. He’s studied military affairs and warfare all his adult life. His popular Breed series of military ad-venture thrillers are admired for combining deep expertise wit/h propulsive action. The premises are realistic, the stories adrenaline-fueled and emotionally engaging.

Check out the books here: Cameron Curtis’s Amazon Page

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