Why my 7.5x55 Swiss PPU Brass is Getting Shorter Over Time?

[ddjchemist]

I have an interesting issue with my 7.5x55 Swiss reloading for my K31 rifle. I use Redding FL die (most recommended die for K31 reloading). My PPU brass is getting shorter and shorter over time. Any other rifle calibers that I reload, brass extends over time as expected and at some pint I have to trim the brass. However, my PPU 7.5x55 brass that I purchased new was originally 2.180". After the first shooting, it went down to 2.176", and now after the second shooting and sizing, it is down to 2.172". The Swiss 7.5x55 brass length range spec is 2.169" to 2.189". It looks that one or two more shooting rounds, and my PPU brass will be too short for reloading. The rifle shoots great and I am getting sub-MOA results (with scope) with my reloads (44.2gr IMR-4064 with Berger VLD 168gr bullets). I do not have any over-pressure symptoms or any issues with the rifle. Does anyone has any idea why my brass is getting shorter over time? I just purchased 50 NORMA cases and am curious if the Norma brass will also get shorter over time. I will reload it for this weekend shooting.

Best Regards from Seattle,

Dan

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[gmerkt]

Well, here's an idea. The shoulder of the case is slowly fire-forming to the shape of the chamber. At some point, it will stop shrinking, then start growing.

If you have any of the new PPU brass left, measure the case diameter at the shoulder. Then measure it in the same place on one of the fired cases.

You may not experience this with the Norma cases.

Re. cases that are slightly under spec. as to length. Not too much of a worry.

 

[JuglansRegia]

That doesn't sound like that is hot enough for brass flow, plus it could be their alloy

 

[gmerkt]

OP loaded 44.2 gr. 4064 with Berger 168 gr. VLD bullet.

Hornady #10 book says 43.2 gr. 4064 max load with their 168 gr. bullet.

Sierra book says 38.2 gr 4064 on low side to 45.8 gr. max load with their 168 gr. bullet.

For reference.

 

[po18guy]

Oversized chamber. The brass is made to minimum spec to accoodate the variation in military chambers. Consider a chamber cast for measurements.

My guess.

 

[ddjchemist]

Thank you all for the input.