Vince I agree with what you had to say also but if I may, let me pick apart as to why? this stuff happens in my opinion.
In any rifle case within a chamber the job of containing the pressure is done by the chamber. Right down to the last quarter of an inch or so when the job passes to the case itself. At this point the brass inside the case should have thickened to take the extra load. It is at the point of handover that a separation will occur if its going to.
The job in my opinion does not pass to the case the last quarter of an inch or so, it can’t because the “brass case” is the weakest link in the whole chain of events. And even extra thick brass will not hold up to 60,000 + psi. So why does this happen?
- It all starts when the chamber is first cut, alignment of the chamber reamer to the rifle bore is extremely critical.
- There is no such thing as a perfectly straight barrel.
- So no matter if you dial in the headstock and tailstock of the lathe to 0.0001” does not mean that the bore “where the chamber is going to be cut” is within the same alignment as the rest of the bore. (so one ends up with a loose chamber down on the case where is shows the most)
So when one compares military rifle chambers to commercial rifle or “match grade” chambers then everything falls apart. Now everybody agrees that “military chambers” are loose or sloppy, why is this? Because of the above stated alignment issue.
I have seen many separations in .303 because of the incredibly loose chambers but every one I have seen is an indication of stress (cold stretching) rather than pressure per se. A rough jagged hairline crack.
The pressure of the round going off is why this “rough jagged hairline crack” occurs because OF THE LOOSE chamber. Not because the brass is so different between 308 Win and 7.62 NATO. Yes that “line” is at the junction point in the case itself (the web) but if the chamber was cut correct or tighter if you will, this is some times not even seen much less felt with a finger nail.
Check it with a mic. and it will be 0.002” to 0.004” if cut correct.
Now “I” am not an expert on this but I have cut a few chambers of all calibers, and I have found that if the lathe is set up as close as one can get that machine to be, you will not have this loose chamber or in some cases “egg shaped” chamber.
I am not knowledgeable in the manufacture of military barrels but I just know in my heart that the operator just sets up the machine “as close as can be” and that’s good enough. And then when that chamber is cut another barrel is set in the lathe right away and on you go. Even the quality of the barrel steel is no where near the same quality as any commercial grade barrel.
And machine gun barrels are worst because it is known from the start that the bore will be shot out very very fast, so why worry about a “loose chamber”
I may have to stand corrected here but, I don’t know of any headspace gauge makers that “don’t have 308 Win and 7.62 NATO as interchangeable” And to put things into perspective here the difference between a “go gauge” and a “no go gauge” is about the thickness of a single layer of “masking tape” ya not much and that is the only chamber dimensions that the military barrel makers seem to care about and sometimes that does not seem to matter much.
So we shooters should have a lot more trouble than we do with this 308 Win. And 7.62 NATO interchangeable truth problem.
Just my opinion and almost not AMMO IAA related.