brass spring back

[bench]

When we fireform or just shoot a normal load and the brass springs back enough to have easy extraction,how much spring back do you think happens to the head space deminsion as compared to the diameter.

The reason I ask is a fired round measures 1.561 so do you think my total headspace is 1.562 discounting any spring back,(measured with hornady)
I do not have any tools to measure from bolt face to chamber shoulder and was just wondering.
Bench

 

[barnesuser28]

If the fired brass has slight resistance when closing the bolt then it is perfectly formed to your chamber, if there isnt any resistance then the chamber is slightly larger than the fired brass but by how much is impossible to know without the reamer specs.

 

[necchi]

how much spring back do you think happens to the head space dimension as compared to the diameter.

Damn little,
"Spring back" is the brass wanting to return to it's original shape after it's been changed and usually full "springback" of diameter can take a full 24hrs.

The matrix of the brass is longer of course in it's length vrs. it's diameter.
So the change has a larger distance (more brass to change) in length. The upset of the larger distance is less (per molecule) than the allowed change in diameter because of chamber dimensions,,
(any one follow that?)

Spring back with the smallest change is more likley to happen than when there is more of the brass matrix to accept the change

 

[fdshuster]

bench: I buy my own chambering reamers (PTG) and have the reamer drawings. What I see consistantly is .001" of springback in the case neck. .269" chamber neck walls will equal .268" as fired diameter.

As far as headspace, that's more difficult to determine with my non machinest measuring tools, and the differences in measurements between my tools and what the reamer maker used.

As barnesuser28 said, I go by resistance felt when closing the bolt on a loaded round, although the FL sizing die is adjusted to bump the shoulder back about .002" with each sizing.

Case head diameter is another issue, and it will continually expand (with some cartriges & loads/ operating pressures), so that problem is kept under control with small base sizing dies, used when needed.

Other than those areas, I'm not concerned about springback. Once the brass is fireformed in my chamber(s) I do whatever is required to maintain those dimensions.

 

[bench]

Thanks fellas great info
Yes that was exactly my point about trying to keep the correct head space as tight as possible without any jam of the shoulder area in the chamber.and the problem with that was I was afraid I was getting some restiance from case head diameter and was hopeing to get a measurement just from fired brass to be within .001 of my chamber headspace (with reloaded round)
Bench

 

[lpreddick]

to really "feel" just the right resistance of the case when closing the bolt, remove the firing pin/spring and ejector and it's spring. i can feel the difference in .001 in. easily and almost think i can detect a .0005 in change. of course case hardening will alter springback and annealing comes into play.

 

[fdshuster]

Bench: And, I should have mentioned: be sure to keep your bolt lugs clean & well lubed. Not saying that you don't, but so many bolts I look at for others rifles are bone dry, & when I mention the need to lube, they act like they never heard of it being necessary.

 

[BoydAllen]

Of course I always keep my lugs clean and greased, and to find out what the maximum attainable shoulder to bolt face dimension is for a given barrel, take an old case and load, neck size and fire till the bolt gets a little stiff, and the shoulder to head dimension stabilizes. Then, I use that dimension as my reference to bump back from, usually .001. If I do not get the feel that I expect from that amount of bump, I know that a different die will be required. If the die is a poor fit to the chamber, too large, setting it by feel will result in more shoulder bump than is desirable. For that reason I always measure.

 

[kraky]

For me I find from the first firing I have about. 002 to go on head space. If you set your FL die for exactly what you see on that first firing you'd be perfect.