[chocdog]

When measuring shoulder length on once fired brass to set up my dies I am getting the following readings 2.040, 2.0405 and 2.041. All rounds are from the same rifle, cases are by Barnes and Federal and I randomly selected 20 rounds.
Are these variances normal?
Should I use the average as a starting point to bump the shoulder?
I thought the the case will form to the chamber, are these differences due to "spring-back" of the brass?

Thanks!

[Striker Fired]

With those kind of numbers your talking about,I find it hard to believe that one can even measure to that kind of accuracy to the shoulder. I assume your checking with calipers and a gage.
.001 is well within any specs on any cartridge anywhere.To attemt to get any closer would be an excercise in frutality.

[gamestalker]

.001" - .002" is about as close to perfect as it gets. But yes, in a perfect world the entire case should take on the exact dimensions of the chamber, but cases spring back when they are fired. Because brass does spring back following high pressures created during firing, some are always going to be slightly different from the other depending on various integrity factors and metallurgy of each piece of brass. The same spring back the brass produces in the gun it, it also does this when run through a die.

[kingmt]

Why take an average? It can't be shorter then the longest case.

[cfullgraf]

With those kind of numbers your talking about,I find it hard to believe that one can even measure to that kind of accuracy to the shoulder.

Some digital calipers will read a ten thousandths digit (fourth decimal place), either "zero" or "five". I have one. I never record the fourth decimal place with my calipers.

[bigedp51]

kingmt

Newly manufactured cartridge case headspace length is allow to vary .007 during manufacture. The actual cartridge headspace length can vary due the spring back of the brass on a once fire case.

On top of this I have seen new cases that were over .009 shorter than minimum headspace with some in the same batch being .002 longer than minimum headspace in the same lot number batch.

http://i122.photobucket.com/albums/o254/bigedp51/308-bjpg.jpg

[Jasper1573]

Yes, those variances are normal. When I resize brass I am looking for a variance of plus or minus .0015. My calipers read out to the .0005, but they only guarantee to the nearest .001, so that fourth digit is nice but really means little when the calipers are only guaranteed to the third digit plus or minus 1.

[kingmt]

I'm not sure why your telling that but I agree.

Unless the pressure is crazy high & expands the case & chamber more then the case can draw back then the largest case is going to be closer to the size of the chamber. As far as I'm concerned 1ths is nothing as long as it still fits.