uncle buck
I seen this and thought you might want to see it. Are you sure you are flatting primers? You might want to chronograph your bullets to see if they are at or even a little past what the books say. A little past or even at max speed can mean a lot in chamber pressure in some guns.
The MYTH Of Flat Primers
Guns Magazine, Oct, 2001 by Charles E. Petty
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Chamber pressure is one of many things in life that are just fine when taken in moderation but destructive when pushed to excess. The problem for the handloader is that we frequently don't know we're in trouble until it's much too late.
The use of primer appearance as an indicator of safe pressure has been thoroughly discredited. By the time primers are markedly flattened, pressures can be considerably above maximums.
As long as we stick with published data everything is fine, but suppose we want to develop a load for a new wildcat, or try a promising new powder in a standard cartridge? Published load data may not exist, but there is a way to find a sane stopping point.
We can do this by measuring the expansion at the very base of the cartridge case, just above the extractor groove. This is the thickest point on the case so expansion here is a good indicator of pressure.
A blade micrometer is the proper tool for this task. You can't make this sort of measurement with conventional calipers or even a micrometer. The anvil (contact surface) on those is much too large to make a measurement on such a small area.
It also has to be capable of measuring to four decimal places. Most calipers only read to three places, or one thousandth of an inch. In order to really get
a clue we have to be able to measure to one ten-thousandth of an inch (0.0001 "). Machinists commonly call these increments tenths.