The importance of case trim

[fiftybmg]

I was zeroing my 308 Norma at 200 meters after adjusting the position of the rings. The load is with a 155 A-Max, approaching the high end of the load spec, and is accurate.

Only the windage was affected by the adjustment, all the shots were around the horizontal center. It took 4 shots to get to zero, and shot five was the final confirmation, which went 4 inches high at 200 meters.

I knew straight away there was a pressure problem. That shot went where the lighter weight monos go at 3400 fps. I suspected that I had made a mistake and dropped too much powder.

Bolt was stuck, but opened with hand pressure, stripped the rim, leaving the case stuck in the chamber. I tapped it out with a cleaning rod.

It measured 8 thou over max trim length. The base of the case just ahead of the belt had expanded over 20 thou.

I pulled all the ammo from the batch, checked the charge weights, and they all checked to within 0.1 grain. Checked case lengths. I had several cases 6-8 thou over trim length.

Mentally retracing the steps, I remembered being a few rounds short for what I needed on the day , so I took half a dozen fired brass, resized, and loaded without checking the case length.

Trimmed, loaded, went back the next day, and completed the zero uneventfully.

Lesson learned.

 

[Mikecr]

So have you measured your chamber end length and determined that trim length was in fact an issue?

 

[J E Custom]

Sounds like you have excessive head space for it to grow .020 in the web area. Case head separation can cause over pressure just like excessive case length.

I would not shoot it until I check the head space and did a chamber cast.

The 308 Norma is very similar to other cartridges, so it is possible it was chambered with the wrong reamer. (This happens more often than people think).

J E CUSTOM

 

[Quicksilver338]

Measure your neck length of your chamber. You'll be surprised how much room you have to grow of saami trim lengths, unless you have specific dimensions to be cut. My chambers have anywhere from .020-.040" of room over Sammi neck lengths.

 

[ar10ar15man]

yep something ain't right

 

[ShtrRdy]

I think the OP is saying he found the problem and the problem is due to case neck being too long. This could pinch the bullet and cause extra pressure to push it out of the restriction.

To the OP - thanks for sharing your experience and reminding us to always check case length.

 

[ar10ar15man]

yes but the numbers listed do not not match typical chamber sizing.
typically 20 plus at neck length
7 or 8 should not be an issue

I think the OP is saying he found the problem and the problem is due to case neck being too long. This could pinch the bullet and cause extra pressure to push it out of the restriction.

To the OP - thanks for sharing your experience and reminding us to always check case length.

 

[fiftybmg]

I have a chamber cast made from when I bought the rifle. The same place ahead of the belt on the chamber cast measure 0.511, the fired case measures 0.516 . From the belt to the case mouth measures 2.335, on the chamber cast 2.332, as near as my eyes can see. This is a point to point measurement, not a true length on the axis.

I've been using the same batch of 150 Norma brass for three years, but I have not kept track of exactly which cases were fired how many times. I've recorded 40 loads in my log book. I've not had a single case head separation.

I will take it to a gunsmith to verify the chamber. Thanks for the feedback.

 

[J E Custom]

I think the OP is saying he found the problem and the problem is due to case neck being too long. This could pinch the bullet and cause extra pressure to push it out of the restriction.

To the OP - thanks for sharing your experience and reminding us to always check case length.

That's Great, But the fact that he had a case head separation stills bothers me. Case head separations are normally a sign of excessive head space that allows the case body to move forward before ignition and grip the chamber walls during ignition and the case head is forced back to contact the bolt face, stretching the case in the web.

A case that Is to long, prevents this from happening, But raises the pressure because it does not/cannot release the bullet correctly.
lowering the pressure will not change the head space and case head separation can still happen over time.

If head space is correct and the chamber is correct and case head separation still occurs, the load was extremely over pressure and is setting the bolt lugs back or stretching the action. Ether one is not good.

Just trying to warn the op that there may be other problems.

J E CUSTOM

 

[ar10ar15man]

chamber cast is only VALID for a very short time after it was cast.
the material changes over time

 

[ResearchinStuff]

Based on what he wrote, and without pictures, I do not believe that he had case head separation, but rather had a severely stuck case. This caused the extractor to either rip off a part of the rim, or to slip over the rim, allowing the bolt to be removed with the case still stuck in the chamber. I know this can happen with a rem 700 style extractor and cases that are too long because 13 year old me did the same thing. I now trim all cases to the trim-to length on every firing, and have never had another issue.

 

[ntsqd]

I'm just happy to see someone else who favors that chamber over all of the later stuff. My first magnum and have soft spot for it even though the 03A3 action couldn't handle it.

 

[sab]

I think tierradelmundo is onto something. The OP wrote:

Bolt was stuck, but opened with hand pressure, stripped the rim, leaving the case stuck in the chamber. I tapped it out with a cleaning rod.

When I read that, I don't read case head separation, I read stuck case with case head intact.

 

[ntsqd]

Never been able to use a cleaning rod to push or tap out a case body after a case head separation (see 03A3 post above). That the OP was able to do so tells me stuck case, not separation.