Headspacing troubles

[Ds J]

Good morning Folks!

When does headspacing turn into a problem?

I have had three or four blown out primers in my 308 over the last year or two. Some readers might remember that I asked about it on other forums.

A month ago, when I was sure all possible causes had been checked and fixed, it happened again that the recoil felt funny, the bolt was stuck and the primer blew out.

Yesterday we determined that the headspacing was too much. The bolt closed stiffly with six patches of masking tape on the back of the case. I also have a small recess on the face of the bolt.

The basics of the rifle and load:

BRNO ZKK601 in 308
PMP cases
CCI primers
36.5gr S335
Hornady 180gr SST
2330fps
1.75mm jump

Any ideas or comments on fixing this?

I have hearf several suggestions:
- fire forming cases
- cutting the barrel and chamber
- replacing the barrel completely

[janfred]

Well, blown primers are usually not a headspace problem unless it is mm's to large. If your firing pin can set off the primer, it shouldn't blow the primer.

Can you place a photo of one of the blown primers?

[Treeman]

6 x masking tape is HUGE head space !, 1 x packing tape, the thin thin one, usually brown in color is usually used to measure this distance non professionally.
6 x masking tape is like worse than most bad .303 are.
I do not want to argue with janfred, but ?? I would blame head space for blown primers.

Next is I would have the barrel set back to rectify.

Blown primers can also result from cases with issues, I get it often when testing beyond max loads and then reload that case - the primer pockets stretch from ++ pressures. 

I would also test to see if this happens across the board with different reloaded cases from different batch's and loads.
 

[Ds J]

Photos to come.

The cases have not been loaded much - less than five times, and mostly with decent loads for practice.

And I am pretty sure that it occurred with another 168gr load and the same batch of  cases.

At one stage I thought it could be magnum primers (I mentioned it on this forum) but that was eliminated.

[Tripodmvr]

@Ds J - please send me a WhatsApp - my phone was stolen and I have lost your number.

I would suggest the following :-
1. Use the existing charge but load with a COL that jams in the lands - QL calculates a pressure of 52K psi for doing that. That should be enough to fireform properly.
2. Anneal 3 cases so that the shoulder is also affected - do this after resizing.
3. You now have a softer font part that can stretch forward.
4. Measure the case lengths after shooting. If there is excessive head space those measurements should show it.

[janfred]

New brass usually has a much smaller base-to-shoulder dimension than fire-formed. If your theory is that excessive headspace causes blown primers, then you should be getting blown primers with factory ammunition and loaded new brass.

A stuck bolt and a blown primer are the classic signs of overpressure. To be sure, check for ejector marks or smears on the case head. Also measure the web expansion. Excessive headspace would not  cause these signs.

An overly large chamber headspace dimension would result in head-case seperations; specially with PMP brass. Treeman may remember this seeing he also owned an old Lee-Enfield. I also had a Lee Enfield No1MkIII* with very generous headspace. I could get 2-3 loads out of PMP brass before getting seperations. Not once did I get blown primers.

The only other reason I can think of is if the case were annealed right through. This would mimic an overpressure load.

[Ds J]

Janfred, we pulled two of the guilty cartridges and the loads were on spec.

We also fired two others after weighing them, and their speeds were on spec as well.

It looks like high pressure, but there is nothing to cause it. If we missed something that could have caused the overpressure I would be delighted to know about it!

[Treeman]

If the primer is not supported about instantly when firing, it will jump out, usually piercing the primer, are the ejected primers pierced or deeper indented?

Initially you spoke of  huge head space ? Go read the GS Custom test method for head space on bolt closing using the tape method.

[Ds J]

https://www.flickr.com/photos/196234911@N08/shares/x9921n2847

The primers were indented at times, and sometimes pushed flat, and blew out three or four times.

The webs of the blown cases were far out of specs.

The chronograph speeds showed now signs of pressure. I got 2330fps from 36.5gr S335.

[janfred]

Are the primer pockets loose on thoses cases?

[Tripodmvr]

Carbon ring? Length of cases?

[Treeman]

https://www.flickr.com/photos/196234911@N08/shares/x9921n2847
The chronograph speeds showed now signs of pressure. I got 2330fps from 36.5gr S335.

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The chroni indicated that the speeds were correct, in that, that the pressures SHOULD be correct, Chroni does not measure pressure, although it can be concluded from velocity.

[Treeman]

Carbon ring? Length of cases?

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There is one I forgot, cases that need trimming, bullet neck getting bitten in the chamber could do this.
I wonder if the shots were still in the group DsJ was shooting.

[Ds J]

Carbon ring? Length of cases?

We checked and didn't see a carbon ring. I will hopefully be able to inspect the rifle in the next week with a bore scope to verify it.

Case length is supposed to be 50.9mm.

[janfred]

Is the primer pockets loose in the cases with blown primers?
Did you measure the velocity of the shots where the primers were blown?
Who loaded the ammunition?
Were the cases annealed by you or someone else?
Have you had any head-case seperations?
What is the case growth per shot?

A carbon ring will give pressure every shot so we can discount that theory.