Sister Cartridges Part 2: Cartridge Variables
Short Story: The cartridge is half of the equation for compatibility between chamber and cartridge. There are several cartridge variables that contribute to the cartridge part of the equation.
Case and Cartridge Geometry
Short Story: The components of the cartridge (including dimensions, weights, and shapes) play an important role in compatibility with a chamber.
The geometry of the cartridge’s case is determined by the features, angles, and lengths of the profile of the case.
- Head or Base: The flat part of the case that rests against the bolt face.
- Body: The main cylinder of the case. This is where the bulk of the powder can be found.
- Shoulder: The transition from the body of the case to the neck of the case.
- Neck: The narrowest part of the case into which the bullet is seated.
- Case Mouth: The open end of the case, through which the bullet is seated.
- Bullet: The projectile.
- Datum Line: This is the approximate midpoint on the shoulder of a necked down case. This is the feature that rests against the shoulder wall of the chamber, preventing the case from inserting further into the chamber.
The cartridge is comprised of the case, bullet, and powder, among other things. The shape and overall length of the cartridge with the bullet properly seated into the neck of the case is an important consideration in compatibility.
5.56/223
7.62/308
For a detailed analysis with references, please see our comparison table with the C.I.P., SAAMI, and STANAG 4172 specifications:
There are no substantive differences between the exterior cartridge geometries of the 5.56 NATO and .223 Remington cartridges; we will consider them to be effectively identical.
For a detailed analysis with references, please see our comparison table with the C.I.P., SAAMI, STANAG 2310, and Kuhnhausen specifications:
There are no substantive differences between the exterior cartridge geometries of the 7.62 NATO and .308 Winchester cartridges; we will consider them to be effectively identical.
Case Thickness and Volume
Short Story: Lower case volume means higher pressure and a more severe pressure spike for the same powder charge. The case volume for the 5.56 NATO and .223 Remington cartridges are approximately equivalent. The 7.62 NATO cartridge has a lower case volume than the .308 Winchester cartridge, due to thicker case walls.
Per Boyle’s Law for a given amount of gas, pressure is inversely proportional to the volume of the container; the smaller the container, the higher the pressure for a given amount of gas; the larger the container, the lower the pressure. When there is less volume for expanding gas to occupy, the container will experience a sharper and more prominent pressure spike.
As such, a cartridge with a smaller case volume may experience higher case pressures and a sharper pressure spike than the sister cartridge loaded with the same powder charge.
5.56/223
7.62/308
We have already determined that the exterior dimensions of the 5.56 NATO and .223 Remington cartridges are equivalent.
Depending on the source of information, you may see slight differences in case volume between 5.56 NATO and .223 Remington (e.g. 5.56 NATO = 28.5gr; .223 = 28.8gr). However, there is nothing inherent in the design of the two cases that would affect the case volume (unlike in the 7.62 NATO versus .308 Winchester discussion). Much of any reported difference in volume can be attributed to slight differences in case thickness from one manufacturer to another, but this difference is not inherent in the design of the case.
For our discussion, we will consider the case thickness and volume of the 5.56 NATO and .223 Remington cartridges to be effectively equivalent.
If you recall, the exterior dimensions of the 7.62 NATO and .308 Winchester cartridges are equivalent.
However, the 7.62 NATO cartridge is heavier than the .308 (7.62 NATO = 180+gr; .308 = 160-170gr) due to the thickness of the case wall. The thicker case wall means the 7.62 NATO case is inherently stronger than the .308 Winchester case. This will play an important part in our conclusions.
As a result of the difference in wall thickness, the interior volume of the 7.62 NATO cartridge is lower (7.62 NATO = 52.0gr; .308 = 56.0gr).
Powder
Short Story: In the same case volume, more powder means more pressure; less powder means less pressure.
More powder obviously increases the amount of chemical energy and potential force within the cartridge. More powder also reduces the free space in the case (see Case Thickness and Volume, above).
| It is worth noting that the temperature of the powder is also important. The hotter the powder, the higher the chamber pressure will be. So, if you leave your hot-loaded ammo sitting out in the sun on a hot day, you can have big problems on the pressure front (even if you are using the appropriate cartridge-chamber combination). |
5.56/223
7.62/308
In the Part 1, we noted that a cartridge fired from a 5.56 NATO chamber will result in lower chamber pressures than if fired from a .223 Remington chamber. This is largely due to the differences in geometry between the two chambers.
To achieve a given chamber pressure, a cartridge fired from a 5.56 NATO chamber must be loaded hotter (i.e. with more powder) than if fired from a .223 Remington chamber. Because cartridges are manufactured toward the upper end of acceptable pressures, a cartridge designed to fire in the 5.56 NATO chamber will be loaded with more powder than one designed to fire in the .223 Remington chamber (and vice versa).
In addition, some 5.56 NATO cartridges are designed to have higher chamber pressures, even when fired from the correct 5.56 NATO chamber (e.g. M855). This can present a big problem for the .223 Remington chamber.
The .308 Winchester cartridge is loaded to higher pressures (i.e. more powder) than the 7.62 NATO cartridge.
Bullet
Short Story: Bullet length can impact the COAL or seating depth, either of which can have an impact on chamber compatibility. However, this effect is not specific to a chamber or cartridge. As such, in order to stick to an apples to apples comparison, we will ignore the effects of longer bullets or differences in seating depths for the remainder of this article. For completeness, we will evaluate the potential effects.
In general, there is no difference in the bullets that can be loaded into a NATO cartridge versus a commercial cartridge. Longer bullets can be loaded into either version of a cartridge. A longer bullet can affect the cartridge overall length (COAL). If a longer bullet results in a significantly longer COAL, it is possible that the bullet may contact the lands of the rifling.
In addition to being longer, bullets can also be seated at different depths in the case for either variant. The depth that the bullet is seated in the case impacts the cartridge in two ways: the free volume in the case and the distance to the lands.
Together, the COAL and seating depth can have a couple of effects when a cartridge is fired:
- The deeper the bullet is seated, the less free space, and the more prominent the pressure spike.
- The longer a COAL or the shallower the bullet is seated, the closer the bullet is to the lands. As we saw in the discussion of the chamber geometry, if the bullet is touching (or closer to) the lands you will have higher chamber pressures and a sharper pressure spike. The magazine of a box magazine-fed rifle can only handle a certain COAL, so this is typically not as much of an issue for the AR platform; if the cartridge fits into the magazine, its probably fine.