5.56 vs. 300 Blackout: Battle for the Best AR Bullet
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5.56 vs. 300 Blackout: Battle for the Best AR Bullet
Joel Nordness Last Updated 2nd January,2023


The venerable 5.56 cartridge, used by the famous M-16 and AR-15 rifles, has seen combat in every conflict the United States has been involved in since the 1960s. While an effective cartridge, there are some cases where 5.56 is less than optimal.

To address the 5.56’s shortcomings and increase versatility, the Advanced Armament Corporation (AAC) developed a new cartridge, the .300 AAC Blackout. The official SAMMI name is 300 BLK, but most casual shooters refer to it simply as 300 Blackout.

With such a distinguished combat history, why was there a need for a new cartridge? What similarities and differences do 300 Blackout and 5.56 have? Read on to learn everything you need to know about these two cartridges!

5.56 NATO

5.56 ammo box

The grizzled veteran in this comparison is the 5.56 NATO round. The exact metric caliber is 5.56x45mm and was developed from the .223 Remington. While sharing the same dimensions, they are not entirely compatible.

Typically, 5.56 has more gunpowder and generates more force than the .223 Remington. With some .223 Remington guns, the area where the rifling begins (the chamber leade) is cut to a sharper angle and could generate excessive force if fired with a 5.56 round, leading to damage. However, .223 rounds can safely be fired in guns chambered in 5.56. While the cartridge loading and chamber leade length differ, the brass cases for 5.56 and .223 Remington are identical.

5.56 cartridge

The 5.56 can trace its origins to 1957, when the U.S. Continental Army Command (CONARC) set forth parameters for a small caliber, high-velocity firearm to replace the M-14, which used a large 7.62x51mm round. A smaller caliber weapon would increase the amount of ammo a soldier could carry, increasing squad firepower and endurance.

The Armalite corporation, who already had the radical new design for the AR-10 in 7.62x51mm, scaled it down and modified it to shoot the new 5.56 round, resulting in the now-famous AR-15 rifle. In 1959, Armalite sold the rights for the two new rifles to Colt, who modified it further, resulting in the M-16.

Entering service in 1964, it first saw combat in Vietnam, but early reliability problems prevented its immediate adoption. After fixes and improvements, the M16A1 was released in 1967, and after proving its increased reliability, it officially replaced the M14 as the standard battle rifle of the US military in 1969. It has served in every major US conflict since.

Colt M16A1 Retro Reissue
Colt M16A1 Retro Reissue

300 Blackout

300 AAC Blackout

Certain military groups noted a performance gap between 5.56 and 9mm Parabellum. Because stealth is important for Special Forces, 5.56 was often a poor solution because they are supersonic and hard to suppress.

9mm, being a pistol round, could be suppressed but lacked the performance of a rifle caliber round.

300 blackout cartridge

Enter 300 Blackout. AAC developed the round to address that performance gap, and the result was a versatile round that is starting to gain favor with both military and civilian shooters.

The round was designed with the AR-15/M-16 type rifles in mind. Using a smaller case but a larger bullet, 300 Blackout is the same length and diameter as 5.56.

This enables 300 Blackout rounds to use the same magazines as 5.56. Because the only difference is the barrel, 300 Blackout barrels have been designed to fit on standard 5.56 lower receivers and can be swapped out in a matter of minutes.

The new round’s versatility is what makes 300 Blackout stand out from the pack. Using a typical 110-grain bullet, 300 Blackout holds the advantage in energy downrange over 5.56. 5.56 holds the advantage at 700 meters and beyond due to its higher velocity and less bullet drop.

It’s the subsonic realm where 300 Blackout shines. The larger diameter of the round allows for a bullet in the 220-grain range to be fired, giving it much more energy than subsonic 5.56 rounds. That is also twice the weight of typical 9mm Parabellum bullets.

Differences Between 5.56 and 300 Blackout

Case Dimensions

5.56 NATO300 AAC Blackout
Bullet Diameter0.224 in (5.7mm)0.308 in (7.8mm)
Neck Diameter0.253 in (6.4mm)0.334 in (8.5mm)
Base Diameter0.377 in (9.6mm)0.376 in (9.6mm)
Rim Diameter0.378 in (9.6mm)0.378 in (9.6mm)
Case Length1.760 in (44.7mm)1.368 in (34.7mm)
Overall Length2.26 in (57.4mm)2.26 in (57.4mm)

As the table shows, the main differences are the length of the case and the size of the bullet. 300 Blackout has a shorter case but a longer and wider bullet with a much less pronounced bottleneck.

The base diameter and overall length are significant similarities, which are essentially equal. This means that, generally, one can load 300 BLK rounds into a 5.56 magazine since the external dimensions are the same.

300 Blackout, 5.56 NATO and 7.62x39 cartridge comparison
L to R: 300 Blackout 125 gr. plastic tip, 300 Blackout 125 gr. match, 300 Blackout 220 gr. subsonic, 5.56mm NATO, 7.62x39mm (AK47 round). Credit: Wikimedia

Ballistic Performance

When fired from an M4 carbine, the NATO standard 5.56 M855 cartridge has a muzzle velocity of 2,900 ft/sec. At a maximum effective range of 500 meters, it has 291 ft-lbs. of energy along with 100 inches of bullet drop.

From the same rifle, a standard 300 BLK round has a slower muzzle velocity of 2,200 ft/sec, but due to the heavier bullet, it has more energy downrange, having the same 291 ft-lbs. at 700 meters, but having the same 100 inches of bullet drop at only 440 meters.

The heavier bullet also gives 300 BLK the advantage at the barrel, with approximately 1,400 ft-lbs. of muzzle energy vs 5.56 at ~1,100 ft-lbs., and the 300 BLK round carries the energy advantage all the way downrange.

Subsonic Performance

Because muzzle energy is a factor of bullet mass and velocity and the speed of sound is fixed, the only variable to play with when it comes to subsonic cartridge performance is mass. Because 300 Blackout has a larger bullet profile, it has a large advantage over 5.56 in the subsonic realm (around 1,100ft/sec.)

A typical subsonic 5.56 round of 112 grains has a muzzle energy of 275 ft-lbs. at 1,050 ft/sec, whereas a 220-grain .300 Blackout round of 220 grains has a muzzle energy of 475 ft-lbs. at 990 ft/sec, almost double the energy. Increased muzzle energy equals increased lethality, an important factor for Special Operations soldiers who want to be quiet and deadly.

Muzzle velocity also depends on barrel length, so a shorter barrel helps keep velocity down. Many 300 Blackout pistols have been released to fill this subsonic niche.

SIG MCX Rattler
SIG MCX Rattler


Because 5.56 has been around for 60 years, many more firearms are chambered in that vs. 300 BLK. However, 300 BLK was designed for compatibility with 5.56-chambered firearms from the start. The nearly identical case diameter and length lets users use 300 BLK rounds in 5.56 magazines, and one can convert their firearm to 300 BLK with only an upper/barrel swap.

There is a risk when it comes to magazine compatibility, though. Because one can use the same mags between the two calibers, one has to be careful to keep them from mixing up if you use both rounds.

Chambering a 300 BLK round into a 5.56 barrel leaves the bullet with nowhere to go since it is larger in diameter than the barrel. The pressure builds up catastrophically, leading to damage or serious injury.

There are companies that make special 300 BLK magazines. Although it’s not needed, if you do tend to mix the two calibers, it’s best to be able to easily differentiate between the two for safety, so it’s best to make sure that your 300 BLK magazines are distinct from your 5.56 mags.

Military Adoption

Dutch maritime special operations force
Dutch Maritime Special Operations Force

Believe it or not, it was the Netherlands that first adopted 300 Blackout as a military cartridge in 2016 when they acquired nearly 200 Sig Sauer MCX carbines for the Dutch Maritime Special Operations Force. The UK Ministry of Defense wasn’t far behind, issuing a five-year tender contract for 300 Blackout supersonic and subsonic ammunition for their forces.

The US Special Operations Command (USSOCOM) also contracted Sig Sauer to acquire their MCX Rattler, which can be chambered in both 5.56 and 300 Blackout. With a small 5.5-inch barrel, it is compact while still firing a powerful round.

While 300 Blackout has several advantages over 5.56, it is unlikely to supersede 5.56 entirely. There have already been millions of 5.56 rifles manufactured, and there are many millions of rounds of 5.56 sitting in depots around the world. It just wouldn’t be cost-effective for the US military to make a complete retrofit.

What 300 Blackout offers, though, is adaptability and versatility. It is easy to convert existing AR pattern rifles to 300 Blackout, and the suppression advantage it offers over 5.56 is large, although suppression is only enjoyed by the Spec Ops soldiers. 300 Blackout fills an important niche but probably won’t be commonly seen by the typical grunt.

5.56 Pros & Cons

  • Better performance at longer ranges
  • Much more common than 300 BLK
  • Cheaper ammunition costs
  • More firearms to choose from
  • Penetration concerns at long range
  • Subsonic rounds lack power
  • Hard to suppress supersonic rounds

300 Blackout Pros & Cons

  • Can use the same magazines as 5.56
  • Compatible with AR-15 lower receivers
  • More energy downrange than 5.56
  • Mixing magazines and chambering into 5.56 firearm can cause damage/injury
  • Higher ammunition prices
  • More bullet drop at extreme ranges


What caliber is 300 Blackout equal to?

300 AAC Blackout is the same as 7.62×35. Its imperial bullet diameter is .308 in.

Does 300 Blackout penetrate more than 5.56?

Performance data shows that 300 Blackout penetrates more out to 700 yards, after which the higher velocity of the 5.56 bullet gains the advantage and penetrates more. For shorter-range engagements, 300 Blackout is superior, while at longer ranges, 5.56 has the advantage.

Which is bigger, a 300 Blackout or a 223?

This question depends on what part of the cartridge you’re measuring. The diameter of the case and base are identical, as well as the overall length of each cartridge, including the bullet. However, the 300 Blackout case is shorter, but the bullet has a larger diameter than 5.56.


The 5.56 round has been in service for the past 60 years and has seen combat in every major US conflict since its introduction in 1964. Designed to be small, high velocity, and penetrate a target out to 500 yards, it has met this role admirably but has its shortcomings.

300 AAC Blackout was designed to address the 5.56 shortcomings, namely versatility in the subsonic realm for Special Operations forces. While much newer and less combat-proven, current performance metrics are extremely favorable, and its compatibility with existing 5.56 magazines and AR-type lowers means adoption is easier.

While currently less popular and more expensive than 5.56, I am hopeful 300 BLK will start catching on and bring prices down.