Case length variation

[usnr2003]

Good morning all,

I did a search on my topic, but didn't see an answer. Sorry if this has been asked and answered before.

I'm relatively new to reloading (9mm) and was prepping some once fired cases for reload. I was measuring using an inexpensive vernier caliper and noticed the length of some cases varied as much as 0.002" depending on orientation. As an example, a case with a reference mark at 12 o'clock measured 0.748", but rotate the case so the reference marker is now at 3 o'clock and the case length measured 0.750".

The caliper measures to the thousandth of an inch, but maybe its not truly an accurate measure? Do I need a micrometer? Is this type of variation expected? I thought this would be a critical measurement as I know the 9mm uses the case edge as a reference in the chamber.

Thanks!

 

[edwardware]

For the 9mm you're loading, I wouldn't bother measuring of trimming. In thousands of rounds, I never have; the only trim length issues I've ever seen were caused by .380ACP or 9mm Largo, both of which are visually separable.

This answer will be very different in different calibers, but 9mm is as simple as they get. Concentrate on not double or zero charging any cases, and enjoy the learning.

 

[Ruger 15151]

Different case lengths are common in both pistol and rifle cases. They both will change with each firing; pistol cases shrink and rifle cases grow. Some pistol cases like 38 special and 357 benefit from consistent case lengths because they require a crimp and the length of the case will effect the amount of crimp each case receives. However, 9mm does not require a crimp so the case lengths aren't critical.

As far as varying measurements around the case, that simply indicates that the rim of the case is uneven. Normally, this would be squared with the case head during trimming. However, its not an issue with 9mm.