Sister Cartridges Abridged: Selecting a Chamber for Your AR

Short Story: You can fire 5.56 NATO or .223 Remington ammo in a 5.56 NATO chamber. Do NOT fire 5.56 NATO ammo in a .223 Remington chamber.

Let’s revisit the relevant facts:

1: See the 5.56 NATO and .223 Remington Specification Tables

2: Per Forster headspace gauge specifications.

From our assessment, we know that:

• The maximum headspace for the 5.56 NATO chamber is longer than the maximum headspace for .223 Remington chamber.
• The length of the freebore of the 5.56 NATO chamber is longer than the .223 Remington chamber.
• The leade angle of the 5.56 NATO chamber is gentler (more gradual) than the throat angle of the .223 Remington chamber.
• The exterior dimensions of the 5.56 NATO and .223 Remington cartridges are effectively identical.
• The case volumes of the 5.56 NATO and .223 Remington cartridges are effectively identical.
• The differences in chamber geometry allow the 5.56 NATO cartridge to be loaded to higher pressures than the .223 Remington cartridge. As such, the 5.56 NATO cartridge is often loaded with a higher powder charge to achieve ideal ballistic performance in the 5.56 NATO chamber.
• The 5.56 NATO cartridge pressures are specified at or above .223 Remington cartridge pressures, when measured in the appropriate chamber.

Next, let’s look at each combination and the issues that can occur:

5.56 NATO Cartridge in 5.56 NATO Chamber (SAFE)

No issue.

.223 Remington Cartridge in .223 Remington Chamber (SAFE)

No issue.

.223 Remington Cartridge in 5.56 NATO Chamber (SAFE)

Because of the more gradual and looser chamber geometry of the 5.56 NATO chamber, as well as the lower loading pressures of the .223 Remington, it is safe to fire the .223 Remington cartridge in a 5.56 NATO chamber.

5.56 NATO Cartridge in .223 Remington Chamber (UNSAFE)

The .223 Remington chamber has a shorter headspace, shorter freebore, steeper leade angle, and narrower throat diameter when compared to the 5.56 NATO chamber. These differences in chamber geometry result in higher chamber pressures when the same round is fired from the .223 Remington chamber versus a 5.56 NATO chamber.

When a 5.56 NATO cartridge that has been loaded to the top end of acceptable pressures for the 5.56 NATO chamber is fired from a .223 Remington chamber, the chamber pressures can reach proof load levels.  For context, proof loads are cartridges that have been intentionally over-pressurized; they designed to exceed the safe operating pressure of a barrel.  Barrel manufacturers fire a limited number of these rounds in a test barrel to demonstrate that the barrel won’t explode if the used with appropriate ammunition, even if the chamber pressures exceed the safe levels for a couple of rounds.  Firing proof rounds continuously would stress the barrel to its limits and could result in explosion of the firearm.  To put the risk in perspective, this is SAAMI’s instruction for use of high-pressure proof loads:

Short Story: It is best to shoot .308 Winchester ammo in a .308 Winchester chamber, and 7.62 NATO ammo in a 7.62 NATO chamber. Do NOT fire .308 Winchester ammo in a 7.62 NATO chamber.

Let’s revisit the relevant facts:

1: See the 7.62 NATO and .308 Winchester Specification Tables

2: Per Forster headspace gauge specifications.

From our assessment, we know that:

• The 7.62 NATO case is thicker than the .308 Winchester case.
• The 7.62 NATO chamber is longer than the .308 Winchester chamber (Min_{7.62 NATO} > No Go_{.308 Win.}).
• The .308 Winchester cartridge is often loaded to higher pressures (i.e. more powder) than the 7.62 NATO cartridge.

Next, let’s look at each combination and the issues that can occur:

7.62 NATO Cartridge in 7.62 NATO Chamber (SAFE)

No issue. This is the safest combination for a 7.62 NATO chamber, especially for older military surplus rifles.

.308 Winchester Cartridge in .308 Winchester Chamber (SAFE)

No issue. This is the safest combination for a .308 Winchester chamber.

7.62 NATO Cartridge in .308 Winchester Chamber (CAUTION)

7.62 NATO cartridges can be longer than .308 Winchester cartridges.  The minimum headspace for 7.62 NATO chamber is above the no-go headspace for .308 Winchester chamber.  If you force a particularly long 7.62 NATO cartridge into a .308 Winchester chamber, the chamber pressures can spike to unsafe levels.  That said, most of the time, you would be fine firing 7.62 NATO cartridges from a .308 Winchester chamber. However, if you are shooting older surplus 7.62 NATO ammo, be aware that the case lengths can create a potential safety issue.

Our advice: don’t shoot 7.62 NATO ammo from a .308 Winchester chamber.

.308 Winchester Cartridge in 7.62 NATO Chamber (UNSAFE)

Because the chamber of the 7.62 NATO is significantly longer than the .308 Winchester, there is more room for the case to expand.  The .308 Winchester cartridge is often loaded to higher pressures than 7.62 NATO cartridges.  Coupled with the thinner walls of the .308 Winchester case, there is a higher chance that the expansion of the thinner-walled and hotter-loaded .308 Winchester cartridge will stretch beyond its limits, exceeding the ultimate tensile strength (UTS) of the brass case.  When the burning powder creates a force in excess of the UTS and the case stretches to its limits, catastrophic rupture of the case results.  Rupture of the case can cause severe damage to the firearm and can cause injury to the shooter or bystanders. Case rupture is particularly of concern if the brass has been reloaded/resized multiple times, as the material will already be stretched more than new brass.

As explained earlier, excessive headspace can also create other issues, like popped primers, light primer strikes, and failure to fire. However, these are matters of reliability more than inherent safety.