Thread: What happens to factory brass on first firing ?

What dimensions would you typically expect to see increase/decrease ? And why ?

Today I fired some new Peterson brass in 6.5 Creedmoor.

Case length : decreased from 1.912 to 1.908
Shoulder datum : increased from 1.553 to 1.560
Web/Base diameter : increased from 0.467 to 0.470

Curious to hear your thoughts and theories

it doesnt worry me.....I will FL resize it and chuck Lee trimmer on it ...if it trims some to bring back to correct length....well and good,debur n champher if not all good debur n champher
its one of those "do I need to worry about" things that I just dont.
if it decreased the brass MUST have flowed outwards....but realistically,why care???

I think it was in Nick Harvey's book I read that the web will expand at the first firing.
All other lengths and widths I would expect to increase or decrease based on differences in dimensions between the unfired case and the rifle chamber - so would vary from rifle to rifle.

Case length decreasing would probably be due to case width increasing, drawing the neck back.

The brass expanding has to come from somewhere so if its not spreading from the web as in your typical start of case seperation then the OA length has to shorten or suck back down as the walls expand.
Kinda goes against brass flow theory though doesnt it.

Other theory is that you cocked up the OAL initially

Neck may get slightly shorter on first firing but case capacity will increase. With longer than spec chamber dimensions in large cases this increased case capacity can change pressures so a load developed in virgin brass may need retuning for second loading. @ebf I am sure you are already aware of the longer term effects of brass flow affecting neck tension as well.

Must have a dud rifle, sell it quick

Originally Posted by Cigar

I think it was in Nick Harvey's book I read that the web will expand at the first firing.

@Cigar, was that "Practical Reloading Manual" ?

Originally Posted by ebf

@Cigar, was that "Practical Reloading Manual" ?

Yes, I think so. I'll try to remember to check tomorrow.

Originally Posted by ebf

@Cigar, was that "Practical Reloading Manual" ?

Yep, couple of bits in the section on load development about measuring web expansion...
"All cases expand noticeably in the base area on firing whether with factory loads or handloads, so this technique cannot be used on the initial firing".
"After initial case-head expansion takes place, which is normal and does not necessarily indicate high chamber pressures, it's advisable to see that case-head do not increase their dimensions after repeated firings".

Greetings ebf,
The case shortening is due to the case expanding to fit the chamber, ditto the increase in shoulder diameter. I really don't think that the web has expanded by .003 inches. I think what you have measured is the case body just in front of the web and this is also just the case expanding to fit the chamber. .003 inches of case web expansion would need pressures well beyond hot loads. Many believe that cases stretch on firing but this is not correct unless the head space is excessive. Cases do stretch when they are run into the FL sizing die. The case gets squeezed down to its original shape and the brass has to go somewhere. Some also think that the case is stretched by the expander button especially if the inside of the neck is not lubricated. This is not correct either. The expander button can pull the neck out of alignment especially if the die sizes the neck down too much but it can't stretch the case.
All in all I think that what you have measured is about what I would expect.
Regards Grandpamac.

@ebf did you measure the base expansion inside the groove with a blade micrometer?

Originally Posted by zimmer

@ebf did you measure the base expansion inside the groove with a blade micrometer?

nope, above the groove with standard outside micrometer

Originally Posted by ebf

nope, above the groove with standard outside micrometer

Just looked up my data for my latest 308 Palma brass, once fired. Measured in the groove in 2 places.
Unfired 0.4053
Same case fired one time 0.4055
Would expect no change after next firing with the loads I am using.

Have seen more movement on other rifles after first firing and then nothing or very little (1/10 thou) with subsequent firings then dimishing to zero depending upon loads.

Did some checks on my 6.5 AI brass and that is more dramatic. 1.3 thou change, and reflected in the primer pocket expansion.
That's after 5 firings with a ripe-ish load.

Greetings ebf,
From your description of using a standard micrometer and measuring above the groove it seems that you have measured the expansion of the case body rather than the case head or web. If you look at Zimmer's post above you can see that his figures for head expansion are a small fraction of the .003 inches you have measured. There are two methods of detecting excessive pressure that I have stumbled across over the years. The first is measuring the expansion of the case body, as you have, and comparing that to the expansion of factory loads in the same cases. The second is by measuring the expansion of the solid case head, as Zimmer does, to detect ongoing expansion resulting in expanded primer pockets and ruined cases. Both methods require knowledge and expertise which Zimmer obviously possesses. I have dabbled with both in the past but currently use a chronograph paired with current pressure tested manufacturers load data for my loading. There is much for you and all of us to learn.
Regards Grandpamac.

Originally Posted by ebf

What dimensions would you typically expect to see increase/decrease ? And why ?

Today I fired some new Peterson brass in 6.5 Creedmoor.

Case length : decreased from 1.912 to 1.908
Shoulder datum : increased from 1.553 to 1.560
Web/Base diameter : increased from 0.467 to 0.470

Curious to hear your thoughts and theories

Don't sweat it you have achieved what you set out to do your cases now conform to your chamber, New brass is always under size so it will chamber in any rifle
no matter how tight the chamber may be, do you have a chamber print from the reamer maker? The reamers I have bought have all come with one, check
the specs to see if they match your fired case.