shoulder length measuring vs pressure- silly question of the day

[rockwind1]

am trying to conceptually understand this. and it's overall importance in regards to pressure and fit in a non-benchrest situation, ie hunting, long range target/ prs type of stuff

as i get better at measuring shoulders with my hornady headspace set up with my mitoyo? caliper

i notice with new brass, i am getting a range of like .002, like in my case almost all are 2.781-2.783,,,sometimes even a 2.784.

on my once fired brass i am getting the same dang measurement! with maybe a few more 2.784's or 2.785's, however 6 out of the 120 1x fired barss was way down at 2.777ish.

now if for the most part i am getting a range of .002, when i shoulder bump .002, how is it really measurable? and does a few thousandth really effect anything as far as accuracy at a 1000 yds ( hoping for 1/2 moa)

i mean if i get a piece of once fired brass that measure 2.781 amd i bump the shoulder .002 so it 2.779,,,then when i put in a case that was 2.783, will it size it back to just 2.781? or will it bring it all the way down to 2.779 ( another .002)? can i really effectivelyl measure it anyway that close?

also, i noticed that my once fired brass (hornady) has essentially the same shoulder length measurement as my new brass (hornady)

getting back to the 6 weird cases,,i am assuming that the approximately 6 once fired cases that measure 2.784-5ish just had softer brass and fireformed more? whereas the others were a little more resistant and need more firings to push the shoulder out?

but what is with the 6 cases that were way short at 2.777ish? this is like .005-007 thousandth less than most of the others?

if headspace is determined by a datum on the shoulder and where it lands on the bevel in the chamber, will this let bolt face push the neck further into the chamber? or will it increase the headspace at the boltface and case head?

 

[Richard Coody]

1 firing doesn't fireform your brass. takes at least 3. don't bump your shoulders until the fired brass dimension stabilizes. A quick way to check is chamber a piece of fired brass. if it doesn't chamber a little tight then it is not ready for a shoulder bump.

 

[rockwind1]

thanks richards, that is actually what i did with the 1x fired stuff. i am proud to say i guess i made the right decision! the bolt closed fine so i figured why? anythoughts on my other mysteries and headspace stuff?

 

[dgeesaman]

New brass expands on the body first, shoulder second. The first firing usually only opens the body to the size of the chamber. Once the body is opened up the shoulder will start to move.

 

[rockwind1]

so if i am full length sizing, then it is less likely the shoulder will move?

 

[BoydAllen]

During the firing of a round, when the firing pin hits the primer, that force and that of the explosion of the primer pushes the case forward in the chamber until the case shoulder is stopped by the shoulder part of the chamber. At this point the primer is sticking out of the back of the case a little and the head is not touching the bolt face (but the primer is). As the pressure rises assuming a dry case and chamber, the sideways force in the case effectively locks the case in this forward position by means of the friction created between the case body and the chamber. The primer is still sticking out and the head is still not touching the bolt face. When the pressure gets high enough the case will stretch at the back and the head will come back to the bolt face, which pushes the primer back flush, in the case of a warm to hot load , and in the case of a medium to light load the head will sort of bounce off of the bolt face a little and the primer will end up very slightly above the head of the case. This and the possibility of primer cratering are why you must either remove the primer or reseat it well below the head before measuring a fired case. The measurement will be off by the amount that the primer may be above the head and by whatever slight amount of crater there may be. When we are dealing with thousandths of an inch these things matter.
With light loads thin brass and smaller shoulder angles a light load can actually shorten a case's "headspace" Those who shoot cast bullet loads in rimless cases need to be aware of this. As the shoulder is progressively moved back by sucessive firings, the neck will become longer faster than the overall case, which will cause it to eventually be crimped into the bullet during firing, by the angle at the front of the neck portion of the chamber. If the shooter is only looking at case length, he is likely not to catch this. The other sign that it is happening is that after each successive firing of a case at that pressure the primer will be sticking out of the case a little more than the time before. This is not conjecture. I have done the experiment....years ago.
Bottom line, in order to understand your measurements you need to understand the forces that have caused them. If we have clearance based on the longest measurement of the datum to head produced in that rifle barrel combination, then we do not need to increase it by setting our die to bump. Bump will vary with brass hardness as will case " headspace". Careful annealing will make all of this a lot more uniform. Even almost new brass can have a lot of difference, case to case in how it stretches and how it responds to sizing.

 

[Bart B.]

Post 8 by Boyd Allen is 100% factual. Well done.

 

[6MMsteve]

Guffey is lurkin somewhere I smell the shoulder bumpinque

 

[BoydAllen]

Post 8 by Boyd Allen is 100% factual. Well done.

 

[jr600yd]

Good explanation Boyd. I was going to respond but you said it better than I could have

 

[Ringostar]

Boy you guys just took Donovans trap and smeared chicken and watermelon all over it!

View attachment 1044724

Tom

So are you calling him Froggy?

 

[Bart B.]

New brass expands on the body first, shoulder second. The first firing usually only opens the body to the size of the chamber. Once the body is opened up the shoulder will start to move.

Then why do all my new cases have more headspace after first max load firing?

They're pushed out by something. Case length is a couple thousandths shorter, too.

Their shoulders are set back a thousandth or two if reduced loads are fired and primers backed out the same amount or more.

 

[carlsbad]

Post 8 by Boyd Allen is 100% factual. Well done.

Somebody must have deleted their post(s) since Boyd's post is now #6. Well done Boyd.

 

[Bart B.]

New brass expands on the body first, shoulder second. The first firing usually only opens the body to the size of the chamber. Once the body is opened up the shoulder will start to move.

All my new rimless case's shoulder headspace grew a thousandth or more when first fired with max loads. Belted ones, too. Evidence to me their shoulder moved forward. All ended up a thousandth or 2 less than the chamber breech to shoulder dimension.

Body diameters grew a couple thousandths or so, too.

Pretty much the same pressure all over inside the case. Wall thickness virtually equal at body-shoulder junction area.

Full length sizing them moved their shoulders forward a few thousandths more before the die shoulder pushed them back.

 

[Bart B.]

so if i am full length sizing, then it is less likely the shoulder will move?

Shoulders will probably be moved forward a few thousandths, may or may not get pushed back by the die shoulder.

 

[ireload2]

You don't have to believe any of these posts.
You can buy a new lot of cases and measure 100 of them both before and after shooting. Once you do that you will have a better understanding of reality.

 

[fguffey]

am trying to conceptually understand this. and it's overall importance in regards to pressure and fit in a non-benchrest situation, ie hunting, long range target/ prs type of stuff

The question is not silly, the answers? The answers are silly. again I was accused of getting into some scary stuff by a smith in Arizona a few years ago. I was testing a few receivers that were advertised as bein suspect. I decided to form cases for a wildcat and test the receivers at the same time. I used a maximum load for a 150 grain bullet 'but' instead of using a 150 grain bullet I use a 200 grain bullet. I listed all of the chamber specifications and the dimensions of the cases. I used one barrel and one bolt to test 4 receivers, there was only .001" difference in the length of the chamber from the shoulder of the chamber to the bolt face.

I explained why there was nothing scary about my methods and or techniques, I explained the time factor, I explained the fire forming load could the maximum load after the cases were formed or the maximum load with the 200 grain bullet could exceed the maximum load after forming.

F. Guffey

 

[yotehater]

 

[ireload2]

What are you talking about guffy???

The question is not silly, the answers? The answers are silly. again I was accused of getting into some scary stuff by a smith in Arizona a few years ago. I was testing a few receivers that were advertised as bein suspect. I decided to form cases for a wildcat and test the receivers at the same time. I used a maximum load for a 150 grain bullet 'but' instead of using a 150 grain bullet I use a 200 grain bullet. I listed all of the chamber specifications and the dimensions of the cases. I used one barrel and one bolt to test 4 receivers, there was only .001" difference in the length of the chamber from the shoulder of the chamber to the bolt face.

I explained why there was nothing scary about my methods and or techniques, I explained the time factor, I explained the fire forming load could the maximum load after the cases were formed or the maximum load with the 200 grain bullet could exceed the maximum load after forming.

F. Guffey

 

[fguffey]

am trying to conceptually understand this. and it's overall importance in regards to pressure and fit in a non-benchrest situation, ie hunting, long range target/ prs type of stuff

Rockwind1, 'pressure and fit' I could not have said it better and then there is the other factor, time. as you can see 'time' is not in the vocabulary of most reloaders. Time is the one factor that prevented me from blowing up the first receiver. The wildcat chamber length from the shoulder to the bolt face was .202" longer that the parent chamber when measured from the shoulder to the bolt face. I know, that used up a lot of case of neck length because the neck shortened from .385" to .215".

I had a maximum load of H4895 for a 150 grain bullet, again I used a 200 grain bullet, I understand that went over most reloaders heads but that load fired in a 30/06 will loosen your primer pockets, shorten the case head from the cup above the web to the case head and it will increases the diameter of the unsupported case head. AND! Depending on what rifle is being used the shooter can expect the bolt to be difficult to open.

I fired 80 rounds, that would be 20 rounds for each receiver, when the trigger was pulled the case had to expand and fill the chamber before the pressure got serious, by the time pressure was serious the bullet was past the beginning of the rifling and by some miracle the shoulder on my cases did not move forward, on my cases the shoulder does not move; when I pulled the trigger my shoulders were erased and became part of the case body and by another miracle another shoulder appeared, like magic! I understand how difficult that is for most reloaders to understand, the shoulders on the fired cases are not the same shoulder I started with, the shoulder on the fired case is a new shoulder.

And then there is the case neck length of .215" on the fired cases, that is down from .385" on the parent case. I know, I am wasting my time to ask a reloader; "What does that mean" but I will ask anyhow; "What does that mean"?

F. Guffey