The 12" twist gun has about 3,000 rounds through it - and shows less throat erosion (through the Hawkeye bore scope) than the 8" twist gun that has less than 2,000 rounds. Reason??
Even though the heavier bullets use less powder to reach full throttle velocities, and the full throttle velocities are lower; the heavier bullet takes longer to get moving down the barrel; exposing the area just ahead of the throat to the forces of particle erosion for a longer period of time.
To add a few more variables to the equation.First have you any way to measure the pressures developed in each barrel being that pressure is a very important factor in throat erosion? Are both bullets the same distance from the lands in the two rifles?I ask this because the closer the bullet is seated to the lands,the more force necessary to engrave the bullet into the rifling which could cause an additional pressure spike at that point and result in more throat erosion.Are both bullets of the same relative hardness for the same reason?Is the longer bearing surface of the heavier bullet(rather than the extra weight) resulting in the resistance to acceleration that is causing the extra throat erosion?Lastly from my experiences,barrels with a quicker rate of twist generally produce less velocity with the same bullet weight than barrels with a slower rate of twist possibly due to the extra work required to rotate the bullet at a faster rate in the quicker twist barrel.Is the faster rate of twist a contributing factor in this case?Since there are so many differences in this case(unknown pressure developed in each barrel,rate of twist,bearing area of bullet,bullet hardness,bullet to lands distance)i would not be so sure that the difference in throat erosion can be attributed to the heavier bullets used in one barrel.