Sister Cartridges Part 4: Chamber-Cartridge Compatibility

Short Story: The case for chamber-cartridge compatibility boils down to pressure. The combination of a cartridge with a chamber can be safe, dangerous, or something in between.

Under Pressure: The Culmination of Chamber and Cartridge

Short Story: You need enough pressure. But too much pressure can be dangerous.

Pressure is your friend. It drives the bullet out of the case, down the barrel, and toward the target. It cycles the action of an AR. Not enough pressure in the right place can lead to low projectile velocities, poor ballistic performance, and cycling failures.

Too much pressure can be your enemy. The extreme pressures experienced in the AR platform amplify failures to a catastrophic level. Over-pressurization can lead to case rupture, violent cycling, excessive wear, heavy recoil, and explosion of the firearm.

The chamber cut into a barrel and barrel extension is designed to withstand a certain amount of pressure. Exceeding the rated pressure can be dangerous. Excessive pressure can result from multiple factors that impact the amount of gas pressure produced (i.e. powder charge and temperature), the volume of the space inside the cartridge (i.e. case volume or bullet seating depth), and the geometry of the chamber (i.e. headspace, freebore length, and leade angle).

Chamber-cartridge compatibility is based on the attributes of the chamber and cartridge, both individually and together as a system.

5.56/223
7.62/308

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:

Factor	5.56 NATO	.223 Remington
Case Geometry	≡ ¹	≡ ¹
Case Thickness	≡	≡
Case Volume	≡	≡
Bullet	≡	≡
Cartridge Geometry	≡ ¹	≡ ¹
Powder	Higher Load	Lower Load
Minimum Headspace	≡ Min: 1.4636″ ²	≡ Go: 1.4636″ ¹
Maximum Headspace	Longer Max: 1.4736″ ²	Shorter Field: 1.4696″ ¹
Freebore Diameter	Wider 0.2260″ to 0.2265″ ¹	Narrower 0.224″ ¹
Freebore Length	Longer 0.0566″ ¹	Shorter 0.025″ ¹
Leade Angle	Gentler 1° 13′ 20″ ¹	Steeper 3° 10′ 36″ ¹
Maximum Average Pressure (Cartridge Fired from Corresponding Chamber)	M193 Copper Crush: 52,000 CUP M193 Conformal: 55,000 psi M855 Conformal: 58,700 psi	SAAMI Copper Crush: 52,000 CUP SAAMI Conformal: 55,000 psi

1: See the 5.56 NATO and .223 Remington Report

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 shoot .308 Winchester ammo in a 7.62 NATO chamber.

Let’s revisit the relevant facts:

Factor	7.62 NATO	.308 Winchester
Case Geometry	≡ ¹	≡ ¹
Case Thickness	Thicker	Thinner
Case Volume	Lower 52.0 grains 3.38 mL H₂O	Higher 56.0 grains 3.64 mL H₂O
Bullet	≡	≡
Cartridge Geometry	≡ ¹	≡ ¹
Powder	Lower Load	Higher Load
Minimum Headspace	Longer Min: 1.6355″ ²	Shorter Go: 1.6300″ ²
Maximum Headspace	Longer Max: 1.6455″ ²	Shorter No Go: 0.6340″ ² Field: 1.6380″ ²
Freebore Diameter	≡ 0.0.3095″ to 0.3114″ ¹	≡ 0.310″ ¹
Freebore Length	Longer 0.1563″ ¹	Shorter 0.1138″ ¹
Leade Angle	Variable 1° 25′ 55″ to 5° 42′ 40″ ¹	1° 45′ ¹
Maximum Average Pressure (Cartridge Fired from Corresponding Chamber)	NATO (M80 eq.) Copper Crush: 50,000 CUP M80 Copper Crush: 50,000 CUP M118 Conformal: 52,000 psi M80 Conformal: 57,000 psi	SAAMI Copper Crush: 52,000 CUP SAAMI Conformal: 62,000 psi

1: See the 7.62 NATO and .308 Winchester Report

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 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.

Conclusions: Chamber and Cartridge Selection

When choosing a caliber for your build, you should clearly state your priorities.

If your priority is to have the widest range of options (e.g. if SHTF, you want to be prepared for ammo shortages, be ready if your buddy drops off a pallet of ammo, etc.), you want to go with the worst-case cartridge option that can handle either sister cartridge.
If your priority is accuracy, choose the cartridge that give you that edge, even if at the expense of compatibility with the sister cartridge or weapon system reliability.

If you already have an AR, shoot the appropriate ammo. If you don’t like that answer, change your barrel.

The tables below illustrate the chamber-cartridge compatibility for the two relevant calibers in the AR system.