Brass shrinking?

[Elwood]

Can anyone explain how and why?

 

My 6.5x47 brass has now been fired three times and some cases but not all measure 4 thou shorter at 1.834. The measurment from new is 1.838, it seems rather odd to me, should I be concerned?

[Andrew]

I wouldn't be: at least if they are shrinking in O.A.L. and not headspace datum. Are you trimming every time you reload? ~Andrew

[Elwood]

I wouldn't be: at least if they are shrinking in O.A.L. and not head space datum. Are you trimming every time you reload? ~Andrew

 

Hi Andrew, no I haven't trimmed as I haven't had to yet. I measured the head space and that has grown about 2.5 thou as you would expect.

 

I should add that I am getting very light markings on the very end of some of the necks (chamfer area) but not necessarily on the shorter cases.

[Alycidon]

Case is expanding slightly to suit the chamber. My PPC brass is just the same, never had to trim it even after 8 or 10 firings.

 

A

[silentsoulsleave308holes]

firstly I would trim every time they are sized then they will all be the same length and same amount of grip

 

secondly the fact that the longer ones have marks on the end would have me concerned that they bumping

[Alycidon]

firstly I would trim every time they are sized then they will all be the same length and same amount of grip

 

secondly the fact that the longer ones have marks on the end would have me concerned that they bumping

 

 

Are you saying trim every time the shoulder is pushed back or every time they are neck sized?. I dont think either is a necessary requirement assuming the dies are set up OK. Maybe a check every 3 or 4 loads.

 

When you say bumping I dont quite know what you mean, I cant see the cases hitting the end of the chamber, if so the bullets would be massivly into the rifleing so your problem is easy to detect. I could not see any manufacturer letting a gun out that has a chamber so short.

 

Perhaps a look using a skilled eye with a bore scope might show a machining problem ?.

 

 

A

[silentsoulsleave308holes]

brass varies in thickness a little in different areas, you might get two cases weigh the same but one is thicker than the other lower down but the other thicker up top..............or.....................cases are drawn and vary and so grow differently in length as they are fired and sized

 

for this reason if the cases are not trimmed to the exact same lengths the one case will have more bearing on the bullet than the other and shot to shot consistency at best will vary, at worse groups go to hell

 

 

 

now your other questions answer is as follows..............your bullet is say 308 (for a 308), the part your bullet engages is 308 ish...but your neck after bullet is seated is around 337 with bullet in place

 

now assuming the chamber at the above point is around 339 so the case can seat nicely and giving two thou clearence, this area only continues for a short measurement until the chamber reduces down to the 308 dia cut for the bullet and your cases maybe bumping this

 

hope the above is clear and explains why your bullets wouldnt be "massively into the rifling" ? if not I will try to explain better

 

hope this helps anyway

[Guest dasherman]

Hi Ian, I've measured my 6.5x47 brass, some have been fired 6 times, neck sized only, never bumped or trimmed and the longest I could find was 1.840". Most are around 1.836" or 1.837".

I think new brass is undersize and when first fired expands to fit the chamber [a once fired case is always larger than a piece of new brass] the brass has got to come from somewhere and I think all that is happening is as the body expands its pulling the neck portion back. Trimming new brass is pointless until its been fired at least once because the cases are just not that consistant. You say your headspace has increased, I assume what you mean is the length of the case from the base to a point on the shoulder has increased. The headspace of the rifle has stayed the same but the brass has fireformed to fit the chamber, again the metal has to come from somewhere.

Marks on the front edge of the necks are most likely to be from ejecting the case and it rubbing on the side of the chamber/action as its removed.

Neil.

[Elwood]

Thanks for the reply guys.

 

Neil, I have measured most of the fired cases and like you most are 1.836 to 1.838 with a few measuring 1.840, as you say brass grows, this is why I'm surprised some of it has shrunk by 4 thou.

 

You could be right about the marks on the case mouths, I do have to hold the brass in and extract carefully otherwise I get severely dented necks, which as I'm using a bushing die can be problematic to remove without using a expander, if it keeps happening I will take some photo's which will explain a lot better

I think I will try full length resizing 50 cases and see what the out come is.

[Ronin]

Re dented cases on extraction:

 

I have a long action rem 700 that has had a Sako extractor fitted by Mr Clark, it works very well but the cases insist on striking the windage knob of the scope on extraction (denting the case mouth), like you this is a neck turned chamber and the necks are maybe .013" thick, I solved this by wrapping some foam in electrical tape over the knob, now I dont get dinged necks anymore.

 

 

Have to admit your case shortening "problem" had me flummoxed, but Neils theory certainly holds water when you think about it.....

[col48]

Hi

 

on the extraction side of things, my 6 x 47 has a spring plunger on a coned bolt, this was shoving the case hard on to the inside of the action,and at times even got stuck hard on to the action.this was making extraction very difficult.

i took my plunger out and kept shortening the spring till i got it to extract OK.

 

don't know what action / system you are using but this might help

 

ATB

Colin

[Elwood]

Colin, I'm sure I'm experiencing the same, my old 223 used to do the this, if I keep my finger on the case when extracting the necks come out without dents, but some will still have the light marks on the case mouth, I'm really not sure if these could be caused by extracting or not.

 

I have some load testing to do tomorrow, weather permitting! I will have a closer look at the extracted cases and see what if any differences there are.

[col48]

Colin, I'm sure I'm experiencing the same, my old 223 used to do the this, if I keep my finger on the case when extracting the necks come out without dents, but some will still have the light marks on the case mouth, I'm really not sure if these could be caused by extracting or not.

 

I have some load testing to do tomorrow, weather permitting! I will have a closer look at the extracted cases and see what if any differences there are.

 

best thing to do is take the plunger spring and plunger out and leave them at home, your rifle will not be affected by this,you will just have to reach in and pick your cases out your action.

if there are no marks then you have found the problem.

 

I'm pretty sure this will do the trick.

 

ATB

Colin

[menial 1]

Hi Ian, I've measured my 6.5x47 brass, some have been fired 6 times, neck sized only, never bumped or trimmed and the longest I could find was 1.840". Most are around 1.836" or 1.837".

I think new brass is undersize and when first fired expands to fit the chamber [a once fired case is always larger than a piece of new brass] the brass has got to come from somewhere and I think all that is happening is as the body expands its pulling the neck portion back. Trimming new brass is pointless until its been fired at least once because the cases are just not that consistant. You say your headspace has increased, I assume what you mean is the length of the case from the base to a point on the shoulder has increased. The headspace of the rifle has stayed the same but the brass has fireformed to fit the chamber, again the metal has to come from somewhere.

Marks on the front edge of the necks are most likely to be from ejecting the case and it rubbing on the side of the chamber/action as its removed.

Neil.

 

Aaahh... that sounds like my mystery solved thank you. When I fire new Lapua brass in my .308 it finishes up shorter than when I first measured it before I load it, so I necksize for the second load and fire again and then I have to trim before I can load again. Rifle shoots okay on first load but I expected lengthening when the case is fireformed, had me wondering about this for ages, thought I was doing something wrong.

Pete.