Despite recent attempts to trample the rights of law-abiding Americans, the AR-15 platform is hotter than ever.

Whether you recently bought your black rifle, had one for years, or just finished your dream build, you may be confused about what caliber ammunition to feed it.

Generally speaking, a modern AR-15 rifle will have been built with a barrel labeled 5.56 NATO, or possibly 5.56 NATO/.223 Rem, if yours is, you have choices. 

*Most will shoot both .223 Rem and 5.56 NATO. (*Note: if your barrel is labeled .223 Rem, it should only shoot .223 Rem ammunition through it, as noted by the folks over at SAMMI. Always check your barrel or call your manufacturer to be safe!)  While both cartridges are similar and share the same dimensions, the chambering for them is not exactly the same. Then there are rifles chambered in .223 Wylde, which splits the difference between the two.

What’s that difference? We’ve broken it down to help you choose the caliber that’s best for your needs and your rifle! 

First, some basics

The .223 Rem is one of the most popular and widely available centerfire rifle rounds in the country. It’s great for just about any purpose. So versatile that it can hunt anything from varmints to smallish whitetails, and anything in between. In 1963, Remington introduced the .223  cartridge, which was the sporting version of the round that was being tested for military use. 

5.56 NATO was adopted for the military and saw it's first major use right away in Vietnam. PFC Michael J. Mendoza (Piedmont, CA) uses his M16 rifle to recon by fire. (Photo: National Archives)

However, in 1964, the Army adopted the M16A1 rifle as a standard issue but specified a new 5.56 NATO caliber ammunition for use in it. This similar but newer cartridge had an increased requirement for penetration which was achieved by tweaking the existing .223 Rem cartridge, increasing the pressure, and naming it 5.56 NATO. 

Another difference between the two is that because it’s intended for military use, the 5.56 NATO cartridge incorporates a crimped primer pocket to ensure that the primer doesn’t detach and drop from the cartridge during difficult conditions that one might find themselves in when firing select fire during warfare.

So what’s the difference?

The biggest difference between the two rounds is pressure. Because the 5.56 NATO round was designed for military use, it fires at a higher pressure. When comparing the two ballistically, even using a .223 Rem and a 5.56 NATO round with the same weight, the 5.56 NATO will produce higher pressure when fired. 

Generally, 5.56 NATO will fire at approximately 62,000 to 63,000 PSI, while the .223 Rem round will fire at 50,000 to 55,000 PSI (depending on the manufacturer). Because of this, you can shoot .223 Rem out of a 5.56 NATO barrel, but don’t try shooting 5.56 NATO out of a .223 Rem barrel as it’s not designed for the higher pressure and could fail catastrophically due to the higher pressurization of the 5.56. 

One of the biggest differences between the two rounds is pressure, which is why you should never shoot 5.56 out of a barrel that is marked for .223 Rem only. (Photo:

Additionally, the chamber length on the barrel of a 5.56 is longer than it is on a .223 barrel. The longer length helps with the higher pressure of the round. Because the .223 barrel has a shorter chamber, the high-pressure round may lead to failures and damage to the interior of the barrel, bolt, gas block, etc., as it was never designed to dissipate the additional pressure.  

Ballistic Comparison

When comparing the same grain weight for each cartridge, 55 in this case, the following ballistic statistics can be compared apples to apples. The data below is from Winchester for "white box" ammo, some of the most common rifle ammo in America.

  • Winchester USA (white box) 55 grain 5.56 NATO at 100 yards: Velocity: 2,898 FPS, Energy: 1,025 FT-LBS
  • Winchester USA (white box) 55 grain .223 REM at 100 yards: Velocity:  2,874 FPS, Energy:  1,008 FT-LBS
Whether shooting .223 Rem or 5.56 NATO you can expect it to be soft recoiling and controllable.


Although the differences are slight, they are different which will cause the bullets to hit their target differently when fired from the same 5.56 barrel. While I’ve used both rounds with the same weight for comparison purposes, the military generally uses a 62-grain 5.56 NATO. Using the 62 grain in the same Winchester USA line the velocity comes in at 2,876 FPS, and the energy at 1,069 FT-LBS.

Are there benefits of shooting one over the other? It depends on how you look at it! 

The benefits of shooting .223


  • Mass-produced and readily available for purchase across the country.
  • Often less expensive to purchase when compared to the 5.56 NATO.


The benefits of shooting 5.56


  • A higher pressure round, it offers better performance and deeper penetration in some cases.
  • The crimped primer pocket ensures a sturdier cartridge, leading to less failures. 



At the end of the day, check the barrel of your rifle. If it’s marked 5.56 NATO, it’s safe to shoot either 5.56 NATO and/or .223 Rem. However, if the barrel is marked .223 Rem, it should ONLY shoot .223 Remington. Both are extremely effective cartridges and will get the job done for you. 

revolver barrel loading graphic