Brass "shrinkage"

[Hinnerk]

Recently I purchased an Arisaka type 99 and loaded 40 rounds using virgin PPU brass. I trimmed all the cases to a uniform length before loading and every single one of them was shorter upon first firing. This makes sense if, as I suspect, the shoulder was too far back on the new cases compared to the chamber in my rifle (at least .015" by my reckoning) and the neck was pulled back to accommodate the expansion.

Is this typical? Is the effect load dependant?

[jwest]

I am not an expert on this round - but what about the brass diameter? Did that increase slightly?

[rsrocket1]

I've heard that if you shoot light loads, the primer shoves the case forward and if the pressures aren't high enough, the brass won't expand the case back. In the case of the .308 and 7.62x54R, I have not experience this but it may happen in other cartridges.

Are you shooting full power loads, light plinker loads or moderate loads with lead?

Did you use a headspace gauge before and after to see if that dimension changed?

[fguffey]

Shrinkage? The case did not shrink, you became a fire former, you chambered a loaded round and then pulled the trigger. When fired the case conformed to the chamber; the case filled the chamber and the distance from the shoulder of the case and case head increased in length. When the case body filled the chamber the neck was pulled back.

I have formed and then fire formed cases that shortened .045” from the end of the neck to the case head, at the same time the case increased in length from the shoulder to the case head.

I have fired 8mm57 ammo in an 8mm06 chamber. When fired the case shortened .250”+. The shoulder never moved, it was erased when a new shoulder formed .127 ahead of the old shoulder. Meaning?

F. Guffey

[Hinnerk]

At this point I do not have a headspace gauge. I was hoping that I might use my 8x57 Wilson Gage as a sort of headspace gauge but the radial expansion was too much to fit into that tool. I used only pieces of paper between the stripped bolt face and the case head of the virgin brass as a make shift measure of how much I expected the case to fire form after the first loading. My loads were on the light side. I used from 39.0 to 42.0 grains of IMR 4046 in 1 grain increments with both Hornady .312 RN projectiles and Sierra .311 MK projectiles, both 174 gr. Five rounds of each load and projectile were built. Primers were Sellier & Bellot LR.

F. Guffey seems to have confirmed my observation.

I wonder only if higher loads would cause more stretching of the aft of shoulder than shrinkage of the neck. I would prefer the latter. I wish that Wilson made a 7.7x58 gage.

[Hinnerk]

BTW, I slugged this chrome lined barrel beforehand as .302/.317 (land to land/groove to groove). I noticed no significant difference in the group spread between the projectiles used.

[ar15barrels]

Recently I purchased an Arisaka type 99 and loaded 40 rounds using virgin PPU brass. I trimmed all the cases to a uniform length before loading and every single one of them was shorter upon first firing. This makes sense if, as I suspect, the shoulder was too far back on the new cases compared to the chamber in my rifle (at least .015" by my reckoning) and the neck was pulled back to accommodate the expansion.

Is this typical? Is the effect load dependant?

This is typical.
The case expands to fill out the available space in the chamber.
As the case does this, some of the neck wil be pulled down into the case body, leaving the overall length of the cartridge shorter than before it was fired.

Re-sizing the case in a reloading die will mostly un-do the process, moving brass from the case body/shoulder back up into the neck as the case is squeezed back down closer to it's unfired size.

[Hinnerk]

Thanks, guys. Yeah, I am a new reloader, learning as I go.

[noylj]

Are you comparing post-sizing/pre-firing length to post firing/pre-sizing length? If so, yes, the case will be shorter. They will magically GROW when you size them.

[fguffey]

[fguffey]

Hinnek, I form cases for the Type 99 7.7mmz58 from 30/06 cases. I use a 308 W forming die. I adjust the forming die off of the shell holder.312” + a few. After forming I adjust the full length sizing die off the shell holder .010” then size a case. After sizing I attempt to chamber the formed case. If the case does not chamber I adjust the full length sizing die off the shell holder .005” and then size the case again.

If the case chambers with slight resistance to bolt closing the chamber could be considered go-gage length. After the bolt closes with resistance I adjust the die to size the case with clearance. That would be the magic .002” reloaders refer to as bump.

When I form a 30/06 case to 7.7mmx57 there is nothing about the process that qualifies as bump.

F. Guffey

[Witch Hunter]

listen to Guffy, this is spot on. Brass does not shrink, it conforms to the chamber dimensions. There is a correct way to size brass, that is to size it to fit each chamber.