Shoulder moving back during firing

[Wolfdog91]

So doing load development with my .243. Mossberg patriot.
So took my measurements to help me do my initial shoulder bump off some older fire formed Hornady brass.
Used the Hornady D400 headspace gauge and anvil and got 1.624"off that brass.
Now new using new starline brass and my die (lee die and shell holder) wouldn't let me bump my shoulder past 1.623" prefer 0.003" bumb but always hear 0.001 i# perfectly fine for bolt guns so went with it.
Well went and fired all 50 pieces doing load work up , no noticeable pressure signs . But now when I measured it to see what these fire formed to their reading
1.621.... so the should went back...
Everything chambered just fine at 1.623 so I don't see how this could be much of a problem but.... Idk should I do anything special here or ?
Thanks

 

[piie]

If you measured the width/girth of the brass before and after, your mystery would have been solved. New brass typically needs to be fired a few times before they can be shoulder bumped or properly sized, because they need to stretch both length and width wise to your chamber shape. The converse also sometimes happen, you see brass lengthening when full length sizing to a point before you can start pushing shoulders back. If the brass is too fat for the die, then running it through the die will increase the length, if the round is too thin for the chamber, then firing it will stretch the width and contract the length. Once properly fireformed, these issues will go away

 

[ronemus]

The brass also gets hot when fired and will shrink when it cools, so it is usually a bit smaller than the chamber; otherwise it would be very difficult to extract.

Manufacturers usually make brass to SAAMI minimum dimensions so it will chamber in any firearm, even one with a chamber smaller SAAMI minimum. In my experience, new brass is typically 0.006" shorter than the fired length, and won't be touched by a sizing die until it is fired.

If you measure overall case length before firing, you'll usually see that it shortens as the body is increases in diameter during firing. You can visualize this with a piece of cloth; if you pull on the width the length will decrease. New brass is typically a few thousandths less than SAAMI trim-to length, and will shorten several thousandths with the initial firing; it may take 3 or 4 firings before trimming is necessary.

 

[K22]

You shouldn't have to bump the shoulder the first few firings on virgin cases and maybe not for several firings after that that assuming you have a group of cases dedicated to that specific rifle. Set your F/L die to zero bump, check a few cases in the rifle chamber with the firing pin assembly removed to assess chambering adequacy.

Comments of Measurements:

1. It doesn't take much to create a .001" measurement inconsistency especially the more devices you add to the caliper.

2. For example, I've have seen one situation where the anvil that fits on the opposite caliper jaw was not cut square creating measurement inconsistency.

3. Also, if you don't align the insert and the holder on some of the Hornady bump gauges the same each time it can create some inconsistent measurements. Match marks help. It's one of the reasons I like the Whidden single piece caliber group specific gauges.

4. It takes some experience and technique mastery to obtain consistent bump measurements. for example, consistent thumb pressure on the moveable jaw and rotating the case to obtain a square position. Practice several times to evaluate your operator performance to see if you are getting repeatable measurements.

 

[Coyotefurharvester]

One other physical action can occur during first firing, the shoulder angle can change slightly. This results in the comparator making contact differently from New to fired brass. Contact further down shoulder(angle increase) "shortens" brass measurements. All that really matters, is don't move the shoulders until brass causes the bolt to close with increased effort.

 

[jepp2]

should I do anything special here or ?

You mention a desire to have a bump of 0.003". But realize that is a number for some from the bolt contact length. When you size this batch of brass, set your die high enough so you DON'T bump the shoulder. The shoulder will move forward as you size the case wall inward.

I'm not going to suggest how much bump you need, that is for you to decide. But I know with my sizing method, I am perfectly fine setting the shoulder back 0.001" from the length where I can just feel bolt contact on a sized round.

 

[D-4297]

You mention a desire to have a bump of 0.003". But realize that is a number for some from the bolt contact length. When you size this batch of brass, set your die high enough so you DON'T bump the shoulder. The shoulder will move forward as you size the case wall inward.

I'm not going to suggest how much bump you need, that is for you to decide. But I know with my sizing method, I am perfectly fine setting the shoulder back 0.001" from the length where I can just feel bolt contact on a sized round.

View attachment 1548170

What he said . Good explanation , and Pictures are always worth the words .