What Caliber AR15 Should I choose?  thumbnail image

What Caliber AR15 Should I choose?

Chris Arnet   |   Dec 3rd 2019

Over 50 years since the AR-15s conception and adoption by the US Military, the options that are available to consumers have morphed the rifle into the most modular and customizable small-arms platform on the market today. In the upcoming series of blog articles, we will be discussing making decisions on each individual component of the AR15 series rifle, and how to build the perfect rifle to suit your needs! First things first, let’s decide on the foundation of your rifle, which caliber you want to build around.

While there are many calibers you can run through an AR15, there are three that are significantly more popular than the rest. Each round has it’s advantages and disadvantages, and it’s important to keep in mind that there is no such thing as the perfect caliber, just the perfect caliber for your needs.

Always the most popular, 5.56mm is plenty capable of fulling most roles

5.56 & .223 Remington

Of course, the most popular by far will always be the 5.56x45mm NATO round. Developed in conjunction with the AR15 by Eugene Stoner, the cartridge was designed for a change in fighting doctrine from previous wars. No longer were high powered long-range cartridges the desired load for a rifleman; it has become more important to be able to carry more ammunition of an intermediate offering.

For the civilian shooter, this means having a time-tested platform and versatility more than anything else. Ammunition choices will range from cheap surplus ammo perfect for a day shooting steel, to lightweight varmint rounds, and heavier hunting and match ammunition. You have a wide variety of aftermarket choices for barrels, more than any other caliber, and hundreds of developed loads on the market designed for your exact role.

The 5.56mm is an inherently accurate round, and with premium ammunition, will not have any problem printing sub-MOA groups at distance. Combat effective to 600 yards, talented varmint and target shooters will make shots out to 800 yards with longer barrels and heavier loads. This round is perfect for varmint hunting, home defense, and being able to practice on the cheap. Ammunition is readily available everywhere, and is undoubtedly the cheapest the options.

The Downsides: 5.56mm does have great terminal ballistics up close, but starts to lose it’s muster relatively quickly. You would not want to hunt any medium to larger sized game past a few hundred yards, or you may risk taking an unethical shot. While the round certainly remains terminal out to distance, you will likely find yourself spending the rest of the day tracking your kill through the woods, as you won’t get the same immediate effects a larger round might offer you.

Best Uses: Home Defense, Varmint Hunting, Target Shooting, Training, Close Range Hunting on Small and Medium-sized Game


.300BLK is becoming more and more popular, and is a versatile cartridge.


.300 Blackout

Designed in 2010 by  Advanced Armament CorporationRemington in collaboration with the United States Special-Operations Command (USSOCOM), the .300 Blackout (or .300BLK for short) was born of a necessity for finding a replacement for submachine guns that were quickly becoming outdated. The requirements were to design a round with better terminal ballistics out of a short barrel, that could easily be switched between supersonic and subsonic loadings, and required minimum parts interchanged from the current M4 platform. The .300BLK accomplished this to a T.

In the past 5 years, the .300BLK has exploded in popularity, helped largely by the “cool-guy” factor of being used by special operations, and it’s part commonality with the AR15 market, most manufacturers are offering an option in the chambering now. But is the round for you? It’s all going to depend on your intended purpose.

SOCOM wanted a round that would achieve the majority of it’s velocity by 9” of barrel length, with diminishing returns after that point. They wanted a hard-hitting compact package, and this can translate into civilian application very well. Building a compact truck-gun, or home defense gun? Want a shorter range pig gun that’s easy to maneuver in heavy brush? Planning on suppressing the rifle? This might be the caliber you want to look at. Wanting to print small groups at 500 yards? You would probably be best looking at another caliber.

The Downsides: .300BLK drops quickly at range, and has very similar ballistics to a 7.62x39mm out of longer barrels, but at a higher cost for ammunition.

Best Uses: Self Defense, Close-Range Hunting, Suppressed Use


Even the 7.62x39mm has premium hunting offerings nowadays

7.62x39mm

While the 7.62x39mm has been around since the design and conception of the AK47, it’s popularity has really only caught on in recent years in the AR15 platform. One of the most popular cartridges in the world, it’s success has been widely documented as a combat effective round worldwide through dozens of conflicts and it has also been successful as a hunting cartridge through bolt-action rifles for medium sized game. It’s popularity translates to readily available and cheap ammunition with plenty of variety with proven track records.

Of the three cartridges here, the 7.62x39mm has the hardest hit by a significant margin. Offering 1,550 pounds of energy at the muzzle, it delivers significantly more energy than the 5.56mm or .300BLK’s 1,350 ft-lbs. It also offers the cheapest options with prices as low as $.17 per round, compared to $.26 on average for 5.56mm, and $.50 for .300BLK. That translates into more shooting for the same price, and more time spent at the range.

The trade-off here is a shorter effective range with less inherent accuracy than a 5.56mm. The 7.62mm is not a precision cartridge, nor is it a far reaching one. While it does hit harder at short distances, it’s slower muzzle velocity means that it will begin to drop faster at range. Most people prefer to keep the round inside 300 yards due to drop, and are typically not using it for punching groups on paper. But then, with ammo so cheap, you can afford to keep pulling the trigger until you score the hit!

The Downsides: Less accurate than other options, greater bullet drop at distance reducing effective range for most shooters

Best Uses: Medium Game hunting inside 200 yards, Self Defense, plinking, and cheap shooting

So What's the Best?

It comes down to you, and what you need. The truth is that with modern ammo, and the fact that most shooters don't ever push a rifle to it's limits, the caliber won't matter to them. If you aren't making long shots on small targets, if you aren't suppressing your rifle, if you aren't a big hunter, then it won't matter. Pick what's important, or easiest, for you. The differences for the average shooter truly will be splitting hairs. However, whatever you choose, we have build kits in every caliber for you. Not sure what an 80% lower is? Check out our other blogs about 80% projects!