Top Secret Calibers To Upgrade Your AR-15

The AR-15 is never missed out from the list of most widely used civilian guns. The greatest thing about this rifle is customization.

A large number of built-in components and accessories are available on the market. So you have multiple options to make your AR-15 according to your preference.

One of the toughest choice when building an AR-15 is about the caliber.

What calibers are compatible with the AR-15 rifle?

Which one will serve your purpose well?

Let me help you with this matter.

The following is a collection of the best calibers to go with your AR-15.

Rimfire Calibers

.22 Long Rifle

.22 Long Rifle

The .22 Long Rifle or .22 LR for short is a traditional caliber of rimfire ammunition. The original AR-15 has the same size as .22 LR projectile, making it simple to build a decent rifle.

It doesn't require an additional power source to fire. You can keep using the bolt and magazine which the manufacturer equip for the gun.

The thing is the bolt is entirely self-contained.

The conversions, as a result, look pretty weird. It may even damage your vision for targeting. It is advisable to attach a bolt carrier in the place of the common BCG reticle. The buffer group will turn the functions of .22 LR to a 5.56, which is standard, beginner-friendly.

The shape of the .22 Long Rifle is a bit round on top so the running line is neat and clean. Nickel boron technology helps to increase the endurance, consistency as well as the accuracy of the kits. Plus, the price is very affordable.

Centerfire Calibers Inch Measurement

.223 Remington Or 5.56x45mm NATO

.223 Remington

If you favor the authenticity or want to get the feeling of an old school AR-15, .223 Remington caliber is for you.

Let turn back to history for a moment.

ArmaLite, the manufacturer of almost all gun accessories, was trying to duplicate the AR rifle, inventing a .308 or 7.62 mm gun or something. But they would like a more compact caliber option so the users could get more cartridges.

That's how we have the .223 Remington as it is at the moment.

Still, the effectiveness of the new, small, and light bullet remains in question. Some say the calibers' movement is super quick, they can break through many kinds of obstacles. Others report a far worse performance. They suppose the .223 Remington doesn't do the magic as the traditional 7.62 and .30-06 ammo.

From my own experience, the bullet is not that bad. But it can't be considered exceptionally good, either. The .223 Remington meets the need of practice. Hunting with 5.56×45mm NATO cartridges can impose excessive pressure on the rifle's body.

.300 AAC Blackout Or 7.62×35mm

The .300 AAC Blackout is a carbine cartridge of Sporting Arms and Ammunition Manufacturers' Institute and Permanent International Commission for the Proof of Small Arms. Another name for it is 7.62×35mm as the bullet is an upgrade from 7.62×39mm Soviet cartridge.

The good news is this version proves to be successful progress.

It has single shots and bolts actions. Plus, the design is ingenious.

The .300 AAC Blackout is pretty flexible. It goes well with all sorts of AR components from bolt carrier groups, mags, buffers to lowers, uppers. Almost everything apart from the gas system and the barrel.

This feature is a compensation for the quite expensive price. Instead of purchasing accessories, you will have to pay a bit higher for the .300 AAC Blackout.

7.62×35mm bullets act as strong support for 5.56. It works best in short-range barrels. The perfect range is approximately 9 inches so that the cartridge can reach the maximum potential.

There is also a myriad of ammo for the .300 AAC Blackout. You can choose either supersonic or subsonic for loading.

As for me, 7.62×35mm cartridges are suitable for various purposes. Not only hunting, they can be used as a home defense also.

Such a wholesome caliber to build an AR-15.

Related that, best scope for ar-15 is most important part when you build Ar15. Read more information by click here.

.458 SOCOM Or 11.63×40mm


The .458 SOCOM, also known as 11.63×40mm, has gentle, big, round top. It works for the lower receiver of an AR-15.

Each bullet weights about 19 gram, a little bigger than other calibers. That's why there are only 10 rounds of 11.63×40mm to fit in a 30 round 5.56 magazine.

Still, it is still a powerful cartridge with a supersonic velocity of 1,900 feet per second. The performance is as good as a light .45-70 with a far lighter case. Therefore, the 458 SOCOM can easily handle fast moving targets

11.63×40mm calibers with an AR-15 is a good match for hunting also because of the solid muzzle device. Recoil may be difficult to control but the muzzle system can give you a hand. However, these heavy-duty bullets are not much appropriate for long-range shooting.

Another disadvantage of the .458 SOCOM is great expenses. I expect it to be more versatile at that price level.

Centerfire Calibers Metric Measurement

6.5mm Grendel Or 6.5x39mm


The traditional type of AR rifle has the average shooting range of about 400 yards or less. But thanks to the invention of the 6.5mm Grendel or 6.5x39mm, the furthest point is increased to 800 yards.

The mutual design of 3 architects: Janne Pohjoispää Some, Brennan, and Bill Alexander makes a great difference. It helps to boost the performance of the AR-15. It improves precision as well.

The 6.5mm Grendel conforms to the standard size of the AR magazine. But the bullet is longer and thinner. Taking advantage of the bigger diameter case, this caliber provides extra power each time you fire the rifle.

There are quite many versions of 6.5mm Grendel calibers. The weight varies from 90 to 140 grains. I usually use the 123 grain 6.5x39mm. It travels at more than 2,500 feet per second, which is very impressive.

Other projectiles with lower dragging can have a supersonic velocity of above 1,000 yards. The kinetic energy it carries is more or less 1,000 foot-pounds.

Needless to say, the 6.5mm Grendel is among the most robust calibers to build an AR-15.

An AR-15 with the assistance of 6.5x39mm works best for medium hunting. Hogs or deer are the perfect targets for this rifle.

6.8mm Remington SPC

6.8mm Remington SPC

6.8mm Remington SPC is the short form of 6.8mm Special Purpose Cartridge. It is also called 6.8 SPC, 6.8 SPC II or 6.8×43mm. It has many names because the caliber is a collaboration between Remington Arms and the United States Special Operations Command.

The original plan was to create an improved variant of the 5.56x45mm NATO with larger projectiles. The final outcome is satisfying, except for the slower firing time.

Because the 6.8mm Remington SPC was made in the early 2000s, the mechanism is a bit different.

Developed by the most experienced professionals in guns and rifles, it can easily outperform the younger brother 6.5mm Grendel. The diameter projectile is, indeed, larger and heavier. But, the bullet doesn't tend to fall down when fired.

Plus, it still fits well in the common AR platform. The rifle magazines are kept high-capacity.  The 6.8mm Remington SPC is likely to give more oomph for your AR-15.

The mass production helps to reduce the price a little bit. But it will never be a product that you can buy without seeing the price tag first.

However, it's worth to pay an extra for more efficiency. The shorter M4 barrel gains greater ballistics thanks to the 6.8mm Remington SPC. An AR-15 with this caliber performs 40% better than the one with .223 Remington.

The 6.8 SPC is also flexible. It can be used for various lengths of the barrel.

To get the utmost power, 20 to 24 inches is what you should go for. A proprietary magazine, BCG, and gas system specialized for the 6.8 SPC will support you achieve higher performance.



Last but not least, it will be the biggest mistake of mine not to mention the 7.62×39mm. If you have ever heard of 7.62 Soviet or .30 Russian Short, it is exactly what I'm talking about.

Though it was an old caliber, first appeared in World War II, the 7.62×39mm seems to never go out of fashion.

It is an intermediate cartridge which is bottlenecked without rims. The range for a shooting is not too far but the round can be very powerful, having a little more oomph than the 5.56x45mm. The 7.62x39mm is suitable for hunting within a distance of 300 meters.

This is also a reasonable caliber for beginners in building an AR-15 with personal identity. But the downside is that it requires a special magazine.

You will want to purchase a separate firing pin for the 7.62x39mm as well. The average one is not strong enough so it may be problematic.

To Cut A Long Story Short

Frankly speaking, there are tremendous calibers to build a perfect AR-15. You have no choice but to try and find your favorite.

But to save your time and money, you can begin with my list. I think it is quite enough for you to get an overview of the world of calibers for setting up an AR-15.

Give these above products a try and tell me your opinion.

Learn more: Scopes and Sights resource.

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