The ANSI/SAAMI piezo standard is to measure pressure with the wall of the case in the pressure path.
There are at least two other piezo standards. One measures pressure with the rod through the case, directly in contact with the powder gas. Another measures pressure at the mouth of the case, presumably without brass in the path. Tonight, I'm not quite ready go detangle which standard is which.
5.56x45 ammunition produces higher muzzle velocity than 223 with the same type of powder and the same bullet, so the actual pressure produced by 5.56 has to be greater than that produced by 223.
Lake City 5.56 brass has more case capacity than any other type I have tested, except Federal. I don't think the small difference in capacity is enough to account for the difference in MV.
CUP and PSI are correlated. Both systems show monotonically increasing values with increasing powder, i.e. you add more powder and both CUP and PSI go up. They must therefore be correlated. It gets confusing because both systems have considerable random noise, resulting in some values that seem out of line. But if you graph CUP vs PSI over a broad range, the correlation is obvious. If you estimate PSI from CUP, you'll get an estimate that is almost as good as estimating the PSI reading you'll get from the previous sample of PSI. I did an article on that a few years ago, and took a fair amount of flack. Later on I discovered that Dr. Brownell estimated the relationship at least a few decades earlier.
The OP's piercing/cratering problem might also relate to shoulders being bumped back too far, but the standard for M16 ammunition is thicker primers. Use regular primers, and you'll very often see exactly this problem. #41 primers are standard for the round. Regular small rifle primers are not. Switch to #41 primers, or to CCI 450s, and chances are the problem will vanish.
The M16 has a floating firing pin. It is not unusual at all to find a tiny dimple from the firing pin on the primer of a round that was chambered and then ejected without firing. To prevent accidental discharge when the bolt is released, the system is designed to use thicker primers.
Last edited by denton; 09/02/15.
