Magazine & Throat Lengths

There are two main factors that will dictate the seating depth of your projectiles when used in bolt action rifles. These are barrel throat length and magazine internal length. In different rifles, each may be a limiting factor on the other. That is, a short magazine may limit you from seating the projectile out as far as it can go in the throat, while a short throat may mean that your loaded cartridges are well short of the length that the magazine will allow.

Why is this important? It is important for two reasons:

1.        Optimum accuracy is often achieved with the projectile seated just short of, or sometimes touching, the start of the rifling. This is not true in every case, but can be considered to be a good rule of thumb; and

2.        Highest velocities will generally be achieved with the projectile seated out as far as it can, as it allows both greater initial case capacity for lower pressures, and also more room for slower burning powders.

It is important to note, that with any cartridge, optimum accuracy and highest velocity may be mutually exclusive. Therefore, depending on your requirements, you may decide to choose one over the other, or you may need to strike a compromise between the two.

Throat length (or 'freebore') is that distance ahead of the chamber to the start of the rifling in the bore. In this area, a projectile of full bore diameter can sit in the bore without becoming engraved by the rifling lands. This allows heavy or long conventional projectiles to be seated out, so as not to reduce case capacity. CEB projectiles are sub-calibre until the first driving band and so will not contact the rifling until that point. Most conventional projectiles have a gradual increase in diameter to the point that are full calibre, while CEB projectiles have a sudden jump to full calibre at the first driving band.

Various rifle manufacturers have magazines of different lengths, even within the same nominal action length. For example, a Remington Model 700 short action rifle (for the .308 Winchester family of cartridges) has a magazine length of 72mm (2.834"), while a Sako 85 Medium Action rifle (also for the .308 Winchester family of cartridges) has an internal magazine length of 75mm (2.952"). Depending on the throating of the barrel, the Sako 85 action potentially allows you to seat the projectile out further.

An additional factor with CEB projectiles will be the location of the driving bands, as it must be ensured that for proper neck tension, that at least two driving bands are in contact with the case neck. However, CEB projectiles have been designed with this in mind, so that in most firearms, the location of the driving bands will not be a limiting factor.


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