stagpanther said:
I didn't know erosion was really that fast with close to the lands COL's.
Erosion is mostly a function of the quantity of powder burned in each round and the peak burning temperature reached during the burn. What changes with throat jamming are two things: one is that it takes less powder to reach either a given peak pressure or a given barrel time, and that should reduce throat erosion. But it achieves those objectives at lower muzzle velocity. That may seem counterintuitive, but the higher start pressure from the jammed bullet causes the round to reach peak pressure before the bullet has travelled as far down the tube as would be the case with some jump and a larger charge that produces the same barrel time. The earlier in the peak in the bore, the more running start the bullet has down the rest of the barrel, so it takes less pressure in the rest of the bore to get it to the muzzle in a given barrel time. But that also means lower muzzle velocity for the given barrel time as the running start got the bullet a little ahead of schedule so it didn't need quite as much velocity to make the same barrel time Folks don't like losing muzzle velocity if they are loading for long range, so, instead they often end up looking for a faster barrel time sweet spot that actually runs at higher peak pressure, thereby throwing away the erosion advantage.
The second thing is that the longer your jump, the less difference, percentage-wise, a change of a thousandth of an inch makes to the total jump. I think this is probably why deeper seating produces wider sweet spots.
There are two strategies for hanging onto a narrow sweet spot found close to the lands or jammed into the lands. One is to keep track of base-to-ogive measurements with a reference gauge, whether it be one you make yourself or you use the Hornady type tool. Changes in that number over the first couple of hundred rounds will provide an estimate of how much you lose per round. Then, assuming you use a seating die with a micrometer adjustment, you can lay out a chart based on that erosion rate that rounds it to the nearest thousandth. You then always fill your ammo boxes in the same order, say, from upper left to lower right, like reading, and set your cartridges in the box in that order as you back the seater off the occassional thousandth to compensate.
The other method is the one Mid Tompkins and his clan of international champions uses, and that is soft-seating the bullet out too far and allowing chambering the round to finish seating the bullet. This used case necks sized loose to allow you to move a bullet in them by finger pressure alone. This has the limitation that you have to remember to tip your barrel up or turn the gun so the ejection port is at the bottom or some combination of the two to clear a live round from the chamber as the bullet will likely have stuck in the throat and you don't want to dump the powder into your action when you eject the case.