[Unclenick]

Bart,

I like the rubber yardstick. I'll probably steal that.

It seems to me someone at Precision Shooting sent test loads around to different facilities sometime in the 90's and got so much variance in the copper crusher results that he concluded it just wasn't a very useful measure. Not only was the method funky but it also depended what technician you got on a particular day as to the result. Some guys just have a better touch with a micrometer than others, it seems.

Wm. C. Davis, Jr. mentioned that the same instrument and operator could one day get a very different reading from a lot of reference loads than he normally did, and put the problem off to there being some variable we still hadn't identified, but I think that was overly optimistic. Some other folks have pointed out that SAAMI fails to control for ambient temperature in its test protocol, and that could affect matters too. Overall though, the copper crusher method was just never as good as was originally thought. I don't know if the lead crushers used for shotgun testing and some sub-10,000 psi pistol cartridges were any better.

I would guess that if the same technician fired some reference loads in a crusher on the same day as he fired loads he was checking, perhaps even alternating them with the reference loads, he could probably say he had or had not exceeded the reference load pressures, but that would be about it. The absolute numbers just aren't very meaningful. Any way you look at it, the crushers are obsolete, and the places that still publish crusher numbers are, again, being optimistic. And consider that calibrated crusher slugs do that badly and then that some folks try to glean estimated psi numbers from case head expansion numbers, another metal deformation measurement except without calibrated metal, and you can see why Bramwell found almost 2:1 pressures causing the same measured expansion.

Sometimes physics just doesn't play nice.