BB,
Impossible if it is not done right. I don't moderate pressure to get the desired reading if that is what you suggest here? If a case expands .00025, you would have to use more pressure to get it to read .00025 less than this, same if it expanded .0005 or .001. More pressure flexes the blade to rail joint on the caliper and you can certainly be consistant and feel a difference in pressure used so this does not happen... with some practice. Is it the best way? No. A .0001 mic certainly is best. What I am trying to say though, is that if the case does expand, the needle will reflect it, with or without a line on the dial at each .00025 or .0005 increment. Yeah, you can use more pressure and see the needle move enough to make you second guess the small change you see but, you use consistant pressure and you can't deny that the case just expanded by the amount you see on the caliper. Could it be off a ten thou, or even two ten thou? Yep, but very doubtfully three ten thou if doing it with some care and consistant pressure. What does .0002 - .0003 CH expansion mean in terms of pressure if using a .0001 capable mic anyway? IMO it means very little because it is just another indicator that you are up in pressure but NOT that you know where pressure is AT. It is far too inaccurate and dependant on many other variables to ever be corralated to an actual pressure to use as a sole PSI indicator, so .0002 - .0003 in the realm of things probably means ~ 5-10 Kpsi. Is measuring CHE accurate enough in predicting pressure that ~ .0002 - .0003, or ~ 5-10 Kpsi is even very meaningful? It probably increases the overall error by around ~ 25% - 30% maybe. Are a lot of these figures I state here assumptions? Yep. Measuring CHE to the very best of ones ability with a top grade mic is also a BIG assumption when you try and relate it to actual chamber pressure because there are so many variables. The strain gage systems attemp to minimize these variables, and do so very well.
Anyone is foolish if they DO NOT use MULTIPLE indicators of pressure and instead rely on one indicator alone, and this includes anyone using a strain gage system too.
These are just my opinions based on my experences and anyone reading them should not read them as "definitive", but rather my simple observations with several assumptions thrown in. Something to think about and test for yourself if you so desire.
Stan says:
The bottom line, though, is that in my judgment any measurable web expansion of a properly fire-formed case -- as happens when you fire a factory round in your rifle -- is an indication of excessive pressure.
Stan has been testing with the 43 for years now, but his statement there is really incorrect, although it is conservative and may keep someone from venturing into the red zone, so it's understandable why he says this, still it is incorrect. ANY case will measurably expand across the web (CH) on firing #2 at 63-65 Kpsi, take it to the bank. Most will expand about .0005", even if the chamber's base is cut to less than a .0005 total clearance. The reason for it is that the barrel and receiver ring DO expand when peak pressure is reached, that is a simple fact, and the case will not hold its shape at those pressures... even though they are safe loads.
Don't take my word for it, or Stan's, expert or no expert, think about it for yourself, and test it yourself. I take very few people's word on very few things but, I do consider most everything I hear or read, but that's just me.
BB wrote:
Have you ever been tested with 9 pieces of brass of different sizes and done a blinded/randomized measurement to verify that you can get .00025" with a cheap RCBS dial caliper?
No I have not.
If you'd like, I could number 10 cases, wrote on tape, measure them, send you the only recorded measurments along with the cases so you could re-number them and return them to be re-measured. I'd send you the second set of numbers I recorded along with each case number. You match up the case numbers and you could tell us how good/bad I did? A third party could be in the loop to further validate the findings.
BB wrote:
I make these remarks not to be critical, but to tell others not to depend on interpolation with a cheap caliper when you should be using a good micrometer.
I totally understand, and agree it is best to use a good .0001 capable mic. As you've stated, it's pretty easy to see how repeatable you can be with a cheap caliper by measuring multiple cases. Record them, have someone re-number them and check them again and compare.
Had to run out and didn't get a chance to reply to your previous posts yet. Off again for a bit but back after a while.
