Case capacity change with use?

[NYDAN]

I was doing some case preparation yesterday for the 270 Win. These cases were all Norma cases and had been twice fired. The cases had been used in load development with different bullets, and different powders, and different powder charges. So, the cases had been exposed to varying pressures. I observed that:

- There were various headspaces (from +.001" to +.0025") on the spent cases.
- With the same die setting, there were slight differences (from -.001" to +.0005") in resized head space.
- After resizing, they required differing amounts of case trimming (from almost none to a few thousands).

Am I correct in concluding that:
- Higher pressure loads lead to a greater headspace dimension in spent cases
- Higher pressure loads will lead to greater case trimming
- Cases lose weight with repeated use due to case stretch and case trimming
- Cases shot at higher pressures lose weight at a faster rate than case shot at lower pressures
- Cases that have been fired multiple times will have greater case capacity that cases fired only once or twice.
- Cases will need to be resorted for weight after every case preparation.

Would these changes in case capacity be significant? Would a load have to be tweaked if using the same brass multiple times?

Thanks for your thoughts.

Dan

 

[jimbires]

Dan , I'll give you my beliefs on this . I'll be very interested in reading what other guys have to say .
* brass is elastic , meaning it will spring back . a new case needs to be fired a couple times (2 or 3), in the same rifle , to reach it's fully expanded size . between firings the case should be neck sized only . then after reaching this fully expanded size the case should be resized with a full length or body die adjusted to bump the shoulder back about .002 .
*when I'm resizing my brass and the case shoulder bump is not consistent annealing the brass will cure this . I use redding competition shell holder sets . this way my die gets bottomed against the shell holder every time . if you resize using standard shell holders there will probably be a gap between the die and shell holder . depending on the hardness of the brass will determine the brass case shoulder bump . harder brass will push the die and shell holder apart more than soft brass will . so now you are dealing with brass spring back and also any play in the press and die assembly .
* if your shoulder bump is not consistent your case length will not be consistent . but it seems that brass will grow at different rates , even from the same lot of brass . shoulder angle has a lot to do with brass growth . the Ackley Improved has a flatter shoulder helping to slow brass growth .
* high pressure loads will expand the brass more , or cause less spring back .
* high pressure loads could cause more brass trimming , but also over sizing your brass will cause this .
* definitely brass looses it's weight with repeated trimming .
* I don't think your brass will change capacity enough to be worried about compensating for it .
* brass weight is a very poor way to sort brass . brass should be sorted by water capacity , after being fully expanded and before being resized . .

 

[kraky]

I'd agree with everything you guys have said. It would be my experience that changes in case capacity over multiple firings would be extremely minimal. I would say a fussy shooter would be better off addressing annealing and getting case neck tension back uniform than worrying about case capacity differences.
I'd also add that the chamber of a rifle has alot to do w/how your brass "wears" over multiple firings. I have two 308 bolt actions...one w/a tighter match grade chamber and one w/a very loose roomy factory chamber. The difference in resizing is very noticeable. And in the loose chamber there is quite a velocity diff between light brass w/more capacity and heavier factory brass w/less. In the tight chambered rifle the velocity diffs are almost the same.

 

[longrangehunter]

I'd have to agree with Jim. Great write up BTW.