Comparing fired neck diameter to chamber

[radmcg]

Most of my rifles were bought used or I was to dumb to ask for neck diameter of the reamer. Other than cerosafe casting the chamber. How can I measure chamber neck size? With "spring back" after firing does fired case neck diameter closely approximate chamber neck size? If so is there some factor to compensate for shrinkage after firing or is it small enough not to matter? I assume it would vary pretty significantly depending on caliber. The reason i ask is i am neck turning remington brass in a 7 saum because the brass is so uneven. I did search this but as usual I'm not the best with the search tool.
AHA
Rad

 

[Fuzzball]

Re: Comparing fired neck diameter to chamber

<span style="font-style: italic">"With "spring back" after firing does fired case neck diameter closely approximate chamber neck size? "</span>

The elastic limit - spring back - for most cases is very close to 1 thou in all dimensions. Thus you can add 1 thou to the neck diameter of a fired case and be VERY close to the diameter of the chamber. (And that's why there is no increase in bullet grip with necks much smaller than one thou under bullet diameter.)

 

[radmcg]

Re: Comparing fired neck diameter to chamber

Thanks fuzzball that's what I needed to know.

 

[Deadshot2]

Re: Comparing fired neck diameter to chamber

When I'm checking a chamber using the "fired case" method, I prefer to use annealed cases. I'll anneal a small batch of cases, load, fire, and then measure the fired neck sizes. This gives me max yield with minimum spring back. The more firings with out annealing, the more spring back. May be not be much but it can make a difference, depending on the case and chamber size.