.223 vs. 5.56: What You Should Know About the Difference

By Daphane Cassidy

One of the most common questions among firearm enthusiasts is whether .223 Remington and 5.56 NATO ammunition are interchangeable. While the two cartridges are similar, there are key differences that impact safety, performance and firearm compatibility.

In this edition of Bullet Points, Erik Scarpati, NSSF Digital Media, Director, sits down with Alan Serven and Brian Osowiecki from the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) to break down the differences and answer this critical question.

Key Differences Between .223 Remington and 5.56

1. Chamber Specifications:
   1. 5.56 NATO Chambers: Designed with a longer throat to accommodate higher pressure rounds.
   2. .223 Remington Chambers: Feature a shorter throat, leading to increased pressure when firing 5.56 NATO ammunition, which can be unsafe.
2. Pressure Levels:
   1. The 5.56 NATO operates at higher pressures compared to the .223 Remington. So, .223 Remington ammunition built to the sporting arm standard in a .223 Remington barrel will produce safe pressures when fired in a 556 chamber.
   2. Firing 5.56 NATO or U.S. Military specification ammunition in a .223 Remington chamber can result in pressures exceeding SAAMI limits, posing safety risks. This can overwork the action, damage the buffer tubes, increase bolt velocity and most commonly cause dropped primers, which can get down into the action of the MSR and cause jams.

So are .223 and 5.56 Interchangeable?

Short answer: It depends. While .223 Remington can be safely fired in a 5.56 NATO chamber, the reverse is not true.

Firing .223 Remington in 5.56 NATO Chambers: Generally safe, as 5.56 NATO chambers are built to handle higher pressures.

Firing 5.56 NATO in .223 Remington Chambers: Not recommended due to potential overpressure issues. Always consult the firearm manufacturer to confirm compatibility.

How to Determine If Your Firearm Is Compatible

The first step is always to check the markings on your barrel. Manufacturers clearly mark their barrels with the appropriate chambering, which tells you which ammunition is safe to use. Some modern rifles are marked .223/5.56, indicating that they are built with a 5.56 NATO chamber and can safely fire both types of ammunition.

Ammunition usage guidance for chambers which do not follow the standards of SAAMI or NATO/STANAG can only be made by the manufacturer of those firearms. SAAMI cannot provide any guidance for chamber designs which do not have a known design and pressure limits standardized by a recognized authority such as SAAMI, C.I.P., or NATO.

If you are uncertain, always consult the firearm manufacturer before using 5.56 NATO rounds in a rifle only marked for .223 Remington ammunition.

What Is SAAMI and What Do They Do?

SAAMI, or the Sporting Arms and Ammunition Manufacturers’ Institute, plays a crucial role in setting industry standards for firearms and ammunition. Their primary mission is to ensure interchangeability, reliability and safety by establishing dimensional and pressure standards for ammunition and the chambers that fire them. This ensures that ammunition performs safely and consistently across different firearms.

Interchangeability in Other Calibers

This isn’t the only case where ammunition interchangeability comes into question. SAAMI provides reference materials outlining other safe ammunition alternatives. View Generally Accepted Firearms and Ammunition Interchangeability guidelines here.

The best way to verify that you are using the correct ammunition is to check the head stamp on the
ammunition to confirm that it matches the markings on the firearm.

Some types of ammunition, most notably rimfire and cartridges produced for military use, do not have cartridge markings on the head stamp of the case. In those instances, check the original ammunition packaging to determine its proper designation. If you have any doubt about the designation of the ammunition, you should not use it until you have it examined by a qualified person.

Remember – just because a round of ammunition can fit into a firearm’s chamber, barrel, or action – this does not necessarily mean that it is safe to use that ammunition in that firearm. Again, always verify that the ammunition to be used matches the markings on the firearm.

Firearm and headstamp markings and other references to specific cartridges and chambers may utilize the full official name, or other approved abbreviated names. View Generally Accepted Cartridge and Chamber Names here.

SAAMI’s Role in Standardizing New Cartridges

Another common question is how new ammunition cartridges become standardized. SAAMI does not “approve” new cartridges; rather, they standardize them to ensure industry-wide safety and reliability. To propose a new cartridge for standardization, a company must be a SAAMI voting or associate member and take responsibility for:

• Producing reference ammunition for ballistic testing
• Manufacturing proof ammunition for safety validation of firearms
• Creating inert action-proving dummies for firearm testing

This rigorous process ensures that newly introduced cartridges meet SAAMI safety standards before reaching consumers.

Final Thoughts

Understanding the differences between .223 vs. 5.56 is essential for firearm owners. While .223 Remington ammunition is safe to fire in a 5.56 NATO chamber, the reverse can create dangerous overpressure issues. The best practice is to always check your firearm’s barrel markings and consult the manufacturer if you are unsure.

For more detailed information, visit SAAMI.org, which provides interchangeability charts and industry guidelines to help firearm owners make informed decisions.

 

Daphne Cassidy is a content manager at the National Shooting Sports Foundation.