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Just FWIW, throat erosion (TE) occurs because when the powder in the cartridge case ignites, the bullet does not begin to move immediately. Instead, its inertia keeps it from moving while the pressure forces gas around the bullet, expanding the case neck and rushing out into throat of the barrel ahead of the bullet. That gas, moving at very high speed (Bernoulli's law) acts like a cutting torch on the steel of the barrel and after while will erode the throat area. When that happens, the bullet can skid as it comes out of the case neck; that distorts the bullet so that its release at the muzzle will be uneven and inaccuracy will result. There are ways to minimize throat erosion - some powders cause it more than other, some bullets will reduce it, etc., but there is no way to eliminate it. Fortunately, it seldom becomes a problem for thousands of rounds, and few sporting rifles are fired that much.
There are gauges that measure TE simply by inserting the gauge into the chamber and looking down at the receiver ring to see which number is even with it. Of course, that means that a TE gauge is good for only one type of rifle; one that works with, say, a '98 Mauser, would not work with a British Rifle No. 4 because the location of the chamber throat in relation to the rear of the receiver ring is different.
Jim
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