A quick internet search for AR calibers shows a Wikipedia page with over 40 different rounds developed for the AR 15 platform, in just centerfire calibers. This did not include rimfire or the AR 10 platform. That's a lot of choices to go through if you want to branch out in your favorite long gun. The question boils down to "What are you going to use your rifle for?" In a political climate that keeps saying there is no good reason for anyone to own one of these guns, it's time to show the versatility of American's favorite long gun. To narrow down and simplify the choice, I've categorized them into three areas of use; Home Defense, Hunting, and Target Shooting.
The first thing I tell anyone looking at a different caliber from their norm is to look at the availability and price of the ammo. If you reload or like to be different, some of your caliber choices won't be listed. The harder it is to find the ammo, the more it'll cost, or you'll have to reload. AR calibers are fickle at times, and availability flows with the latest and greatest flavor of the day. Some great rounds are lost in time solely because it wasn't popular. Meaning you'll only get it if you load it yourself. Those of us without that option is going to miss out on some great performing rounds simply because we can't find it. Keep that in mind with your decision.
A lot of people use an AR for home defense and given the right round, caliber, and situation; it's an excellent option. That being said, it's not my personal choice. The 5.56 is the standard AR caliber, and it's a small, fast, and penetrating round by design. More people live in apartments, subdivisions, and neighborhoods nowadays than in the past. If you are living out in the country where your nearest neighbor is 300 yards through trees, then a rifle round meant to be effective out to 500 yards can be a valid choice. However, if you are one of those that can walk out on the front porch and see your neighbor, then you might want to give the 5.56 a second thought. Also, with a smaller size, remember that the impact will also be smaller. So shot placement is critical. For a gun enthusiast, this is an argument that may go on forever. The point is if you choose the 5.56, I highly recommend training and practicing. One nice thing is that 5.56 ammo is readily available and inexpensive, which gives you more opportunities. Also, choose the right ammo for your home. I cannot tell you how many people over the years tell me about their AR for the house then show me a mag full of the fun ammo with green tips, making me very happy I'm not their neighbor. You are responsible for every round that leaves your gun, and the last thing you want to do is use a penetrating round that has a higher chance of collateral damage. Your neighbor isn't the one threatening you, so be responsible and choose ammo that expands and opens to create a wound change and transfers enough kinetic energy upon impact that makes a pass through less likely or less lethal. If you want 5.56, one great example of defense ammo is the Hornady Critical Defense. Made for closer engagements, lower velocity impact, and controlled expansion, this round is ideal for home defense.
An option I like is an AR in a pistol caliber like 9mm (a PCC). These give you the same benefits of a handgun round along with the same ergonomics and recoil reduction of an AR. Keep in mind that in a 16" barreled AR, the pistol round will be traveling faster than out of a handgun thereby giving it's effective range a big boost. However, a PCC gives you an option of a larger diameter round, than the 5.56, which creates more kinetic energy and damage upon impact. One feature I'm a big proponent of. Like the 5.56, the ammo choice is vital. An FMJ 9mm will penetrate through the target and into what's behind it. Choose a good defensive hollow point that will expand and transfer that energy to keep collateral damage to a minimum. With any of your choices, I'd recommend testing these on the range to see their true effectiveness. Private ranges are great for this because you can put out a target, like a melon, and something behind it. This allows you not only to see what the hollow point does to your target but what happens to the object behind. Test it out at realistic close ranges too, to truly understand its effectiveness.
The anti-gunners call the AR a "weapon of war" and say "It's only meant to kill people," "You can't deer hunt with it" and many other things that show their lack of knowledge. For many years the .223 Remington in bolt rifles has been a go-to round for varmint hunting like a coyote (a round that runs in a 5.56 chambered AR). It would make sense that the AR in the same caliber to become the next evolution. Several manufacturers, like Rock River Arms, have developed specific platforms designed to be the ultimate predator rifle. With the ARs multi-caliber versatility, there have been a significant number of options developed for hunting purposes, no matter the game. Meaning this long gun can be used for hunting and more than just battle. Another great round, that's a favorite for varmints in my area, is the .204 Ruger. It's flat shooting, accurate and found at most hunting stores, like Cabelas. Also available to be chambered in your AR, this would be my top choice for coyote hunting in even long-range venues.
Coyote hunting has been a popular use for the AR for many years. However, what about whitetail hunting? This is considered the most sought-after big game in the US and is part of almost every state's history. Is the .223 a good round for whitetail? It can be. A friend of mine has used a .223 bolt action rifle for whitetail does for a few years now, without issue. With proper shot placement, it can be a good round for the job. However, many will argue (including myself) that a larger round is more ethical and can be more forgiving of less than perfect accuracy. Fortunately, the AR comes in over 40 calibers. I'm a fan of a 30 caliber for whitetail, so I look for something in that arena. The .300 Blackout is the first to come to mind. It is a shortened.30 caliber round developed by AAC for use with a suppressor. It has become prevalent over the last few years and is available at most gun stores. Hornady is a go-to manufacturer for these rounds for hunting purposes. They make several different loads ranging from 110 grain to 190-grain bullets with good expansion to be used to take big game. With the ballistics of 2,215 fps and transferring 1,360 ft-lbs of energy in a 125-grain bullet, it is highly effective for a 150 to 200 lb animal out to about 500 yards.
There are many other rounds in the AR platform that are suited, some even developed, for big game hunting. Another of my favorites is the .450 Bushmaster. A very hard hit, heavy round that is easy to shoot and is ideal for big game. It's a .45 caliber, straight walled cartridge developed from the concept of the "thumper" by Jeff Cooper. It is very flat shooting out to 200 yards and transfers 2,700 ft-lbs of energy keeping roughly 75% of the round's weight as it passes through. Extraordinarily useful and will drop any big game it hits (including black bear). It's had a resurgence lately with a demand for straight wall calibers in some Midwest states. These are just a couple of options, my two favorites for where I live, but there are more. This is also just the AR 15 platform of rifles. If you want to step up to the AR 10 style rifle, you get the ability to use common deer rifle rounds, like the .308 Winchester. Either way a completely viable option. One thing is obvious; the AR is completely capable of being used for hunting and is done so every year all over the US.
The most common use for an AR is target shooting. Let's face it, a day at the range is fun and with an AR, even more so. Whether you are just plinking with a .22 LR or testing your distance abilities with something like the .224 Valkyrie, you are having a great time. With so many varieties of calibers, it's hard to choose which is the best. No matter what your preference is or your end use, practice makes perfect. Ultimately all the calibers are suited for practice and target shooting, so get out and shoot. For this article, I'm going to focus on calibers ideal for a day of burning powder and hitting targets.
The best all-around plinking caliber will always be the .22 LR. It's cheap, easy to shoot, and can be found everywhere. There are several uppers and inexpensive ARs that make this caliber a wonderful option. The full rifles will run you in the neighborhood of $400 and the uppers about the same. When 500 rounds of .22 can be purchased at your local box store for $30 or less, this is an excellent caliber for a day of fun. Training with a .22 has also been proven to increase your skills. While not the felt recoil of a centerfire round, a .22 AR has the same function and size as the standard giving you the ability to focus on handling and trigger basics, which are the most important aspect of the gun.
If you are a long-range shooter, the AR is fully capable of 500 plus yards with amazing accuracy. Several calibers fall within this foray, many that also have counterparts in the bolt action world. Ballistically the .204 Ruger and .224 Valkyrie are running top dog in this world. They are known as flat shooting, hot loads that can reach out. Another option would be the 6.5 Grendel. A predecessor of the 6.5 Creedmore, it has outstanding long distance capabilities and is large enough to take big game. Any of these rounds will make a day at the long-range fun and can truly fulfill your desires to reach out. There is a downside to these rounds. They aren't cheap or as common. While this style of shooting is becoming more and more popular, it is not an everywhere kind of round. Stores like Cabelas will more than likely have them in stock or you can find them online. However, your mom and pop shop or Wal-Mart may not keep them in-stock.
If you want to get more into your favorite rifle platform and its capabilities, changing your caliber is an excellent way to do so. Besides the popular options that I've listed, there are a ton of others. Some are more obscure, and some are just out of popularity. These can be a better option for your needs, but you will need to look into reloading. There are some very cool options to play with and loads that you can work up to best match the capabilities of your gun. If it interests you, then I'd highly recommend getting started. However, for the rest of us, the calibers I listed are considered the best options. Take some time to decide what your gun is going to be used for and then go from there. After choosing the caliber, remember to practice. Get out and shoot. It's the best way to get the most out of your gun, and it's just plain fun.