Case head separation with new Browning X- bolt 300 win mag.

[QuietTexan]

I would have necked up with a 338 cal mandrel, used a neck die back to 308 cal until they snuggled up nicely, and enjoyed the extra case capacity

 

[Bob Wright]

My vote is bad chambering. I've never separated a 7RM case and with .019 fireforming to the shoulder.

 

[Reelman1]

Chip ringed chamber, maybe. I have seen it and actually ruined a barrel while chambering one.

 

[Hugnot]

I headspace my .300 WM on shoulder. I adjust my die using a .007 feeler gauge between shell holder & F/L die bottom., Like, why the belt? - a useless antiquated feature designed for straight or cartridges having shoulders that are almost flat.

I never have had a head separation in my .300 WM. Upon stripping the bolt (no firing pin or spring), I feel a slight resistance upon closing the bolt.

 

[jammer300]

Oh wow
I hope they do you right. Something is very wrong in that chamber

Thanks. I'm hopeful too

 

[quigley257]

Very common for belted mags to stretch a LOT on first firing just above the belt. If you use a case comparator to measure virgin brass versus once fired brass, the amount of movement on the shoulder datum line can be huge. I like the Peterson "long" 300 Win Mag brass for this exact reason. While your rifle may indicate perfect headspace while using SAMMI spec gages, the measurement from the bolt face to the shoulder datum line can vary wildly from manufacturer to manufacturer on belted rounds. This is easily seen by measuring a handful of factory 300 Win Mag ammo with a case comparator and noting the differences in length(to the shoulder). It is not uncommon to have well over .015" increase in base to shoulder length on the first firing. All that stretching has to occur somewhere, and it usually manifests itself just above the belt as shown in the OPs photo.

 

[jammer300]

Chip ringed chamber, maybe. I have seen it and actually ruined a barrel while chambering one.

Interesting. I have never heard of this but will do some looking into it for sure. Thanks.

 

[Pdvdh]

Very common for belted mags to stretch a LOT on first firing just above the belt. If you use a case comparator to measure virgin brass versus once fired brass, the amount of movement on the shoulder datum line can be huge. I like the Peterson "long" 300 Win Mag brass for this exact reason. While your rifle may indicate perfect headspace while using SAMMI spec gages, the measurement from the bolt face to the shoulder datum line can vary wildly from manufacturer to manufacturer on belted rounds. This is easily seen by measuring a handful of factory 300 Win Mag ammo with a case comparator and noting the differences in length(to the shoulder). It is not uncommon to have well over .015" increase in base to shoulder length on the first firing. All that stretching has to occur somewhere, and it usually manifests itself just above the belt as shown in the OPs photo.

What you're saying is true for a beltless cartridge, that headpaces off the shoulder. Yes, the case stretching you refer to largely occurs at this web location on a beltless casing that headspaces off its shoulder, if headspace is excessive.

On a belted casing, properly headspaced on its belt, the stretching occurs almost entirely in the shoulder area. Not at the case web.

The OP says he's seeing impending case head separation with once-fired and twice-fired cases. On a belted cartridge properly headspaced against the forward belt, the cartridge case head is held against the bolt face. The case head can't push back toward the bolt face when chamber pressure builds. So the case shoulder blows forward to fill the void space between the case shoulder and the chamber shoulder. The case stretch occurs in the shoulder, which is the most malleable, thinner, and weaker portion of the casing.

In order to stretch the case wall significantly at the web, where these cases are separating, the case head has to move back toward the bolt face after the casing side walls have gripped the chamber wall tightly, as internal case pressure climbs.

I have only seen belted cases exhibit impending separations at this location if headspace on the belt is excessive, and after multiple firings. Not in 1 or 2 firings.

I'm afraid I agree with with MagnumManiac on this. Who was a practicing gunsmith for 40 years, and exceptionally proud of it.

 

[victor3ranger]

Interesting. I have never heard of this but will do some looking into it for sure. Thanks.

I have a picture of this in the AR section

 

[jammer300]

I have a picture of this in the AR section

Thanks. I'll check it out

 

[MagnumManiac]

Very common for belted mags to stretch a LOT on first firing just above the belt. If you use a case comparator to measure virgin brass versus once fired brass, the amount of movement on the shoulder datum line can be huge. I like the Peterson "long" 300 Win Mag brass for this exact reason. While your rifle may indicate perfect headspace while using SAMMI spec gages, the measurement from the bolt face to the shoulder datum line can vary wildly from manufacturer to manufacturer on belted rounds. This is easily seen by measuring a handful of factory 300 Win Mag ammo with a case comparator and noting the differences in length(to the shoulder). It is not uncommon to have well over .015" increase in base to shoulder length on the first firing. All that stretching has to occur somewhere, and it usually manifests itself just above the belt as shown in the OPs photo.

This common belief is entirely untrue! The cases do not stretch at the web, the shoulder moves forward to fill the void, the base of the case is held firmly in position. If what you say is true, then a case would separate on the second firing in every rifle made. When fireforming AI cases, where does the case move? At the shoulder.
Please look at this picture, where is there ANY stretching above the web on this several fired 300WM case?

 

[huntinfool18]

Measure the diameter of a new case vs a fired case where the line is. Could be that the chamber got polished to much and your case is growing to much during firing.

 

[quigley257]

This common belief is entirely untrue! The cases do not stretch at the web, the shoulder moves forward to fill the void, the base of the case is held firmly in position. If what you say is true, then a case would separate on the second firing in every rifle made. When fireforming AI cases, where does the case move? At the shoulder.
Please look at this picture, where is there ANY stretching above the web on this several fired 300WM case?

Clearly not as the case is properly headspaced to the chamber.

 

[jammer300]

Measure the diameter of a new case vs a fired case where the line is. Could be that the chamber got polished to much and your case is growing to much during firing.

Where the line is on the fired cases measures .511" and the unfired cases measure .507".