At the start of 2009, I had a couple of 13.5 tw barrels chambered for my Panda. having tested and found the magic seating spot for boattail bullets at 7thou off the lands, I had a good measure of success shooting point blank benchrest locally.
After about 400 rounds, I found inconsistencies in group pattern (larger than accepeted group sizes).
My question would be: is throat erosion directly relevant to chamber pressures generated ? and; as such, how often should you adjust seating depth ?
Any change in seating depth will no doubt alter the pressure curve (and possibly the accuracy) slightly.
The erosion of the lands is of course changing the seating depth. This happens quicker in the first 500 rounds, as the leading edge of the lands 'rounds off' (all be it only slightly).
I use one of those Stoney Point things to check the jam point, and if necessary make adjustments. One of the beauties of seating into the lands, means that this is not so critical. I used to seat 8 thou off the lands for a few years, but have now gone back to "jam plus 15 thou" in all of my barrels.
Works for me.
Brendan,
Thanks for that.
I would like to better understand your comment of 'jam + 15thou.
Is it the initial setting of your bullet say 'jammed' into the lands, and then further advance the seating by 15thou?
I first establish the point at which the bullet contacts the lands - that is, gives a witness of four land marks (Kreiger and Bartlein). I then set the seating die for this point, and add a 15 thou shim to the seating stem.