Q: I set out to measure the capacity of four .30-calibre cartridges based on how many grains of water each case would hold and got the following results: .308 Win, 56.5 grains; .30-06, 71.3 grains, .300 Win Mag, 93.7 grains; and .300 Weatherby Magnum, 104.5 grains.
The .308 and .30-06 cases were Federal, the .300 Win Mag was Winchester Super X, and the .300 Weatherby was Weatherby.
I used once fired cases and filled them to the case mouth, but a self-styled expert at my local rifle range said I should have measured case capacity to the base of the bullet. What do you think?
– Donald Martin
A: I think your so-called “expert” is full of prunes. Case capacity is sometimes measured to the base of the bullet, but this is a variable involving attributes of the rifle and bullet design; it also depends on magazine length and throat length.
I’ve also known others who liked to measure case capacity to the base of the neck.
However, bullets aren’t always seated to the base of the neck; sometimes they’re seated deeper than the base of the neck. Measuring case capacity to the case mouth relates specifically to the case and eliminates the variable concerning firearm and bullet design.
A critical factor is that the Weatherby chamber has a long throat which acts to reduce pressures because a bullet has to move some distance before engaging the rifling. The point of peak pressure occurs during initial bullet acceleration and engraving. In a standard calibre with a normal length throat where these processes occur simultaneously, pressures are greater than if one occurs prior to the other. The long throat allows the bullet to accelerate before it encounters the resistance of engraving.
Looked from another angle, a bullet can be seated farther out in a long-throated rifle to effectively increase powder capacity.
If you’ve been reading my articles for any length of time, you’ll know that I have had a long throat cut in most of my personal rifles, which allows me to seat bullets out so they will just fit in the magazine, and they all shoot very accurately.
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