Getting head around headspace

[bowji john]

I have measured the length  of chamber using the hornady headspace kit - 

Headspace measured at 1.768 using a fired PPU case and insert C375

I then started the process of reloading using new Lapua 6.5 x 55 cases

I was surprised to discover that my now once fired Lapua cases measured only 1.762 using the same C375 insert

Even the same Lapua, now twice fired, is measuring at 1.72 to 1.764

I'm not knocking back the shoulders at all as I'm probably getting a head space of 1.768 minus 1.762 = .006 of an inch 

My questions are 

1. Since we are measuring outside dimensions why might there be such a discrepancy between the PPU and the Lapua?

2. with a headspace of .005 to .008 am I risking over stretching the web and risking case head separation with my Lapua

3. Will the measurements of the Lapua settle down and conform to those originally measured (using PPU) with further firings

4. Despite taking great care with the reloading process I am getting a velocity spread of around 60 fps - might that variation be in part due to the slight differences in head space as not all the cases measure quite the same and I can't uniform them because I dare not bump the shoulder back even more?

[Big Al]

53 minutes ago, bowji john said:

I have measured the length  of chamber using the hornady headspace kit - 

Headspace measured at 1.768 using a fired PPU case and insert C375

I then started the process of reloading using new Lapua 6.5 x 55 cases

I was surprised to discover that my now once fired Lapua cases measured only 1.762 using the same C375 insert

Even the same Lapua, now twice fired, is measuring at 1.72 to 1.764

I'm not knocking back the shoulders at all as I'm probably getting a head space of 1.768 minus 1.762 = .006 of an inch 

My questions are 

1. Since we are measuring outside dimensions why might there be such a discrepancy between the PPU and the Lapua?

2. with a headspace of .005 to .008 am I risking over stretching the web and risking case head separation with my Lapua

3. Will the measurements of the Lapua settle down and conform to those originally measured (using PPU) with further firings

4. Despite taking great care with the reloading process I am getting a velocity spread of around 60 fps - might that variation be in part due to the slight differences in head space as not all the cases measure quite the same and I can't uniform them because I dare not bump the shoulder back even more?

Forget what you have measured with a PPU case if you are now using Lapua brass. Make all your measurements with the same brass just to rule out any possibility of misreadings.

I assume you are depriving cases before measuring as small primer craters can give you false readings?

Out of interest what is the rifles action?

[bowji john]

Yes I am de-priming first using a universal de-primer

Blaser R8 pro

[bowji john]

Yes I am de-priming first using a universal de-primer

Blaser R8 pro

[bowji john]

Yes I am de-priming first using a universal de-primer

Blaser R8 pro

[Big Al]

Are you having any problems chambering once fired Lapua cases?

If not I would measure a batch of say 20 and record the length, its very rare I see a difference of even 0.001" between 20 twice fired cases. 

Sorry I dont have any experience of Blaser actions or how to feel for an exact case fit.

I think your initial measurement with the PPU case is throwing you off the scent a bit, if your fired Lapua cases are coming out consistent then thats where I would be taking my measurements from. 

I assume you are rotating the case while in the comparator so it settles before taking a reading?

[bowji john]

Cheers Big Al

I'm going to ignore the PPU and concentrate on the Lapua

I've a batch of once fired that I'm reloading at the moment - thereafter I hope to see consistency on second firing

It has been suggested elsewhere that with a combination of the elasticity (spring back) of the Lapua brass and the low pressures associated with 6.5 x 55 swede (estimated via 'Quickload' to  be a peak pressure of 50470 psi) that the case is not expanding into the chamber fully.

This might be rectified on 2nd or 3rd firing

[bowji john]

Cheers Big Al

I'm going to ignore the PPU and concentrate on the Lapua

I've a batch of once fired that I'm reloading at the moment - thereafter I hope to see consistency on second firing

It has been suggested elsewhere that with a combination of the elasticity (spring back) of the Lapua brass and the low pressures associated with 6.5 x 55 swede (estimated via 'Quickload' to  be a peak pressure of 50470 psi) that the case is not expanding into the chamber fully.

This might be rectified on 2nd or 3rd firing

PS - I don't understand why I'm getting multiple entries of my comments

[gunner]

Off topic but it could be a glitch in the matrix .