As I understand it (I'm an electrical engineer, not a metalurgical engineer) from reading and talking to some of my metalurgical engineer friends, the process that wears out the throat is a combination of thermal shock errosion and abrasion (from several things).
Any meaningful discussion has to begin with a description of the process that causes the erosion in the throat. Somewhat simplified, the blast of hot gas (above the melting point of the barrel metal) heats the metal on the surface of the throat and immediately beyond cherry red to a depth of a a thousandth of an inch or possible a bit more. The exposure to the gas doesn't last very long, milliseconds, and it is very hot (couple thousand degrees), and under high pressure. That combination heats just the surface of the metal. An eighth of an inch or even a sixteenth of an inch deep there is a temperature change but nothing like the several hundred degree change on the surface. It is this temperature gradient from cherry red on the surface to only a bit warmer a few thousandts of an inch deeper that casues the errosion.
The metal on the surface expands with the flash of heat, then cools very rapidly as the heat source goes away and it loses heat rapidly to the much much cooler amd more massive collection of barrel metal immediately adjacent to the hot layer. The differential expansion of the surface metal with respect to the immediately adjacent metal causes thermal stress, a shearing stress between the layers of metal. This stress cycle is repeated with each shot, and will eventually cause a thermal fatigue failure of the metal on the surface.
Metal will flake off having fatigued to the point that it is no longer attached to the metal underneath. The surface will start to look like it has scales. The surface metal expands and distorts itself with respect to the layer underneath, then shrinks which with enough repititions causes scales to form when it contracts. It will look almost like the surface of a dry lake bed when viewed through a borescope. I think I have some pictures of this at home that I've taken in my shop. I made a setup to use my Nikon 990 with the Hawkeye borescope to take pictures.
It's a cumulative thermal shock fatigue effect. The more damage done per round the shorter the barrel life.
The other mechanism is abrasion, wear, of the throat and barrel by material coming out of the cartridge - powder, bullet, burnt powder, and so forth. More of this causes more wear than less.
OK, with that basic understanding of the mechanism, one can draw some qualatative conclusions about what will make it worse or better:
- Anything that increases the rate of change of temperature on the surface will increase erosion. Hotter gas temperatures, faster temperature rise, will increase erosion per round fired. Faster powder will generally be hotter at the throat than slower powder. More powder will increase the depth of the hottest layer by keeping it hot a millisecond or two longer.
- Lighter bullets with fast powder may be harder on the throat than heavier for caliber bullets and slower powder.
- More powder hurts two ways - more material to grit blast the throat area as it moves into the barrel and sustained heat to increase the depth of the heat affected zone.
So the cartridges that would be expected to have shorter barrel life would be those that have a lot of powder relative to the bore volume because they increase the thermal shock and dump more grit into the throat. A 7 mmMag will be harder on the throat than a 7mm-08. A 7mmBR will be easier on the throat than a 7mm-08. The .308 will have longer barrel life than a 7mm-08 (but probably not a lot longer) because of the larger bore which allows the gas to get past the throat sooner with the same weight bullet. I'd expect a .338 Federal to last about forever, but maybe not as long as a .30BR.
I'm not at home right now, so I can't give you a link, but there are a couple of spreadsheets available on the WEB that proport to predict useful barrel life. Playing with them, at least qualatatively they show variations as a function of powder choice (burn rate), bullet weight, and powder charge weight, that are at least consistant with this theory - i.e. the changes in barrel life are in the direction expected based on the mechanism described.
A .30BR has amazing barrel life. If my memory is right, national matches have been won shooting barrels with 6,00 to 7,000 rounds through them. The relatively small powder charge, and relatively large bore volume compared to case volume is all in the right direction to promote barrel life and more than overcome the light for caliber bullets usually used in a .30BR. If you want a rifle to practice with that will shoot well for thousands of rounds, get a .30BR.
Anyhow, that's how I understand it. I have no indispensible ego attached to this so if you have a different theory, please put it out there so we can have a dialog about it.