Sister Cartridges Part 1: Chamber Variables
Short Story: The chamber is half of the equation for compatibility between chamber and cartridge. There are several chamber variables that contribute to the chamber part of the equation.
Headspace
Short Story: Optimal headspace is long enough to allow reliable feeding/extraction, but short enough to contain the pressures and expanding force.
Headspace is the distance between the face of the bolt and the chamber feature that limits the insertion depth of a cartridge placed in it. This barrel feature complements the case geometry between the base/head of the case and the datum line, an imaginary line about halfway up the shoulder of the case.
Headspace is an important consideration in the design of a barrel, but is generally not something that you need to obsess over if you are buying a quality, commercially-available barrel (and you know what to order…).
A proper headspace is tight enough to support the case during the firing cycle. The pressure generated inside the cartridge when the primer detonates is substantial; in fact, it is enough to stretch the brass case to conform to the chamber. Too long, and you can have some issues, including:
- The case will stretch more than it should. Overstretching cases (especially if repeated due to reloading) can result in case head separation or rupture, which can cause damage to a firearm and injury to the shooter. In general, use caution firing commercial reloads, because you have no idea how these cases have been treated over their lifespan.
- The cartridge may sit too far into the chamber and the firing pin will not strike the primer with sufficient force (“light primer strike”) to reliably ignite the cartridge.
- Because the bolt face will not adequately support the base of the case, primers may blow out of the primer pocket from the pressure that builds when fired. These loose primers can get lodged in weird places, including in the chamber or fire control group, and cause cycling issues or failures.
A proper headspace is loose enough to allow the round to fully seat into the chamber and allow the bolt to close, even if the chamber is dirty. If the headspace is too short, the round will not seat properly (especially if the chamber is dirty), which can cause a number of issues, including:
- The bolt will not close. This is not, technically, a problem by itself. Except…
- Failure to close the bolt means you will not be able to fire the gun. If you have read our Mass and Gas article, you know that the bolt rotates as everything goes into battery. The cam pin rides the angled groove in the bolt carrier and cannot rotate until the last moment of the travel cycle. As the bolt carrier group is travelling, the bolt extends forward in the bolt carrier. The cam pin can only be in the “extended” (vertical) position as it rides through the upper receiver. In this forward position, the firing pin cannot protrude beyond the face of the bolt. The prevents the firing pin from accidentally striking a primer while the bolt is out of battery. As the bolt carrier group lands in battery, the cam pin rides the groove into the locked position, the bolt rotates, and the locking lugs on the bolt engage the locking lugs on the barrel extension. In this closed state (bolt in the rearmost position within the bolt carrier), the firing pin can now protrude from the face of the bolt when struck from the rear. If the bolt does not seat far enough back in the bolt carrier, the firing pin will not protrude. No protrusion, no bang.
- You may be able to force the bolt closed. If sufficient force is applied, the neck/shoulder of the case may be compressed rearward or crimped around the bullet, which can lead to significant spikes in pressure (the same amount of gas in a smaller space results in higher pressure; added “hold “grip” on the bullet will require more pressure to unseat it).
One principle is conserved in the sister cartridges discussion: As with most things designed for field use, the military designed the chamber with reliability in mind. In the field, weapons get dirty; a soldier doesn’t have the luxury of breaking down his or her service weapon after few rounds to clean it because it might be a little dirty. As such, NATO chambers are generally designed with looser tolerances. A little extra room doesn’t do accuracy any favors, but a cycling gun is more important in this context. The tighter chambers of commercial variants means less tolerance for dirt and fouling and a tighter fit for cartridges; this tighter fit can compound with other variables that impact chamber pressure.
| It is worth noting that most current AR-10/LR308 barrels are chambered in .308 Winchester. Unless you are shooting a vintage AR-10 (i.e. manufactured by Armalite or Colt decades ago), you probably won’t be using an AR barrel cut with a 7.62 NATO chamber. As such, to the AR interests, this is less of a “choose this chamber”, and more of a “for this chamber” discussion. If you are shooting a vintage AR-10, M1A, or M14, we’ve still got you covered. |
5.56/223
7.62/308
The minimum headspace for 5.56 NATO and .223 Remington chambers is the same: 1.4626″. However, the maximum headspace for 5.56 NATO is substantially longer than the max (field) headspace for .223 Remington (1.4736″ vs. 1.4696″).
Not only is the maximum headspace longer in the NATO variant (as is customary for military chambers), but the minimum is significantly longer. In fact, the minimum headspace for the 7.62 NATO chamber is longer than the No-Go headspace of the .308 Winchester.
Chamber Geometry
Short Story: The freebore length and throat angle can have substantial effects on chamber pressures. A longer freebore is associated with lower chamber pressures. A shallower throat angle is associated with lower chamber pressures.
Headspace (the distance between the datum line to the bolt face) is a simple concept. However, what happens forward of the datum line is more subtle and complicated. We can break the chamber down into a few important sections:
Looking a little bit closer, we can see some very subtle features:
The Freebore, or Throat, is the space between the mouth of the case and the start of the rifling. This is the area of the chamber where the bullet does not contact the rifling. Technically, the Collar also meets the definition of “freebore”, so many diagrams include this in the measurement. The larger the diameter of the freebore, the more easily the bullet will move. The longer the freebore, the more the bullet can move, unrestricted, before it encounters the lands of the rifling. It should be noted that the freebore of some chambers is tapered; this means that from the collar to the bore, the chamber diameter is constricting.
Beyond the freebore, the chamber begins to constrict; as you progress forward, the lands are gradually exposed. At the point where the full height of the lands is exposed, we find the start of the bore. This transition between the freebore and the bore (the gradual exposure of the lands) is called the Leade. The angle of this transition, the leade angle (or throat angle), effects how quickly the bullet is introduced to the resistance of the rifling.
The widest portion of the bullet is a bearing surface (a surface that contacts the rifling as it travels down the bore of the barrel). A portion of this bearing surface protrudes from the case and may contact the freebore when the cartridge is chambered. The distance between the forwardmost bearing surface of the bullet and the start of the rifling is called the throat jump or bullet jump. This is the length that the bullet can travel, unrestricted, before contacting the lands.
Effects of Freebore Length
The longer the freebore length (and throat jump), the more forgiving of variation in the overall length of the cartridge (whether due to bullet seating depth or bullet length/shape, in general) and the more reliably a round will feed into the chamber in dirty (i.e. practical) conditions. A shorter freebore length and throat jump length usually contributes to better precision/consistency and better accuracy. Striking a balance in throat jump length is effectively striking a balance between accuracy and feeding reliability.
It is worth reiterating that military chambers reflect the reality of war. A long time ago, the military rightly opted for reliability (i.e. longer freebore) over accuracy (i.e. shorter freebore) in their service weapons; the benefit of 2 MOA over 3 MOA at the expense of operability was not an acceptable compromise for service personnel.
Before we discuss the effects of freebore length on pressure, we should understand inertia and its role in the equation. A moving object has inertia (the product of mass and velocity), and that inertia is proportional to the velocity (and mass) of the object; the faster an object is moving (or the heavier an object moving at a particular velocity), the more kinetic energy it has. The more kinetic energy an object has, the more easily it will overcome resistance. If an object cannot move (or can barely move) before it meets resistance from an external force, the velocity component is negligible or null, so the kinetic energy much lower (or null). This means that overcoming resistance occurs primarily via the impulse that sets the object in motion, and the force required to overcome the added resistance is greater than if the object was in motion. You see a similar phenomenon in the current of a motor: getting the rotor in motion from a standstill requires a pulse of extra energy (this might be identified as “starting current”, “inrush current”, or “surge current”). If that rotor was already turning, even a little, the surge current would be substantially lower.
The effects of freebore length on chamber pressures can be substantial. The shorter a freebore, the less a bullet can move before encountering additional resistance from the rifling (i.e. less of a “running start”). In extreme cases (with short freebore + long bullet + shallow bullet seating), the bearing surface of the bullet can actually contact the lands when loaded into battery. In these scenarios, chamber pressures can be significantly higher and the pressure spike (just like the “starting current” of a motor) will be more prominent.
Effects of Leade Angle
The steeper the leade angle, the more abrupt the transition from freebore to bore. All else being equal, a steeper leade angle means the bullet meets resistance more suddenly, which can result in higher chamber pressures and a sharper pressure spike. When coupled with a shorter freebore length, a steeper leade angle can contribute to a legitimate pressure concern.
5.56/223
7.62/308
For a detailed analysis with references, please see our comparison table with the C.I.P., SAAMI, STANAG 4172, and Pacific Tool and Gauge specifications:
The 5.56 NATO chamber is designed for practical field applications. In the field, rifles get dirty. Things aren’t always perfect. As such, the 5.56 NATO chamber is a little more forgiving in a couple of areas:
| Specification | 5.56 NATO | .223 Rem. |
|---|---|---|
| Freebore Diameter | 0.2260″ to 0.2265″ | 0.224″ |
| Freebore Length | 0.057″ | 0.025″ |
| Leade Angle | 1° 13′ 20″ | 3° 10′ 36″ |
| Leade Length (trigonometric) | 0.3322″ to 0.3513″ | 0.0922″ |
- The freebore or throat diameter of the 5.56 NATO chamber is slightly larger (0.2260-0.2265″ vs. 0.224″).
- The freebore or throat length of the 5.56 NATO chamber is longer (0.057″ vs. 0.025″).
- The leade or throat angle of the 5.56 NATO chamber is less abrupt (1° 13′ 20″ vs. 3° 10′ 36″).
The difference may not be obvious from the measurements, above. In a simplified and overlayed diagram, you can see the slight difference in leade geometry (length and angle):
The overlay shows the difference. It may not seem like much, but it’s enough to compound with the other differences in chamber and cartridge to affect the observed chamber pressures.
It is worth noting that the combination of longer freebore length and shallower leade angle of the 5.56 NATO means lower observed chamber pressures for a given load. This allows the 5.56 NATO round to be loaded hotter (with more powder) than the .223 Remington sister cartridge, without exceeding the maximum chamber pressure when fired from the appropriate chamber. Remember this for later.
For a detailed analysis with references, please see our comparison table with the C.I.P., SAAMI, STANAG 2310, Kuhnhausen, and Pacific Tool and Gauge specifications:
There is a lot of variation in the 7.62 NATO chamber specifications, but there are some notable differences between 7.62 NATO and .308 Winchester chambers:
| Specification | 7.62 NATO | .308 Win. |
|---|---|---|
| Powder Chamber (Bolt Face to Neck) | 1.711″ | 1.7039″ |
| Case Length (Bolt Face to Collar) | 2.029″ | 2.025″ |
| Freebore Diameter | 0.3085″ to 0.3095″ | 0.3098″ to 0.310″ |
| Freebore Length | 0.156″ to 0.168″ | 0.09″ |
| Leade Angle | 1° 25′ 55″ to 5° 42′ 39″ | 1° 45′ |
| Leade Length (trigonometric) | 0.095″ to 0.3246″ | 0.3222″ |
- Because of the longer headspace, the 7.62 NATO chamber is slightly longer in the powder chamber and case length measurements.
- The freebore or throat diameter of the 7.62 NATO chamber is roughly equivalent. On paper, the 7.62 NATO chamber trends a bit tighter (0.3095″ vs. 0.310″).
- The freebore or throat length of the 7.62 NATO chamber is significantly longer (0.156-0.168″ vs. 0.09″).
- The leade or throat angle of the 7.62 NATO chamber is highly variable, but can be much more abrupt (5° 42′ 39″ vs. 1° 45′). It is important to note that this is the opposite seen with the 5.56 NATO and .223 Remington chambers.
The most significant of these comparative measures is the freebore length. The longer freebore is the primary factor in the pressure equation.
In the 5.56 NATO and .223 Remington discussion, we saw that the leade angle was an important contributor to the equation. However, that is due to combined effect of a steeper leade angle AND a shorter freebore length. While the 7.62 NATO chamber can have a steeper leade angle, the effect is rendered negligible when paired with the longer freebore.