Author: MaxP
Subject: Brass life expectancy ?
Posted: 18 November 2013 at 1:41pm
You have missed the point, I'm afraid. Once fire formed (neck sizing only) your cases no longer headspace on the rim, but on the shoulder the same as a rimless case. If you do the right thing by your cases on first firing, then rimmed headspace has no effect after that.
The chamber is a taper cut by a reamer with a set angle. It's diameter dimensions will be proportional to the depth of the shoulder. In a long chamber, the diameter at the base of the cartridge will be greater, and allow greater expansion.
The case sitting in the bottom of a "generous" chamber is a problem. Unless the head gap is .000 or less, the case will not be centred by the bolt face on closing. Once struck by the striker, it is in fact nailed into position, held on the bottom surface of the chamber and only allows brass expansion in one direction- toward the top of the chamber. This is what causes your concentricity issues you are able (due to having a tight-ish chamber, {but obviously not tight enough if the cases are not coming out concentric}) to reduce by rotating your cases 180deg for the second firing.
If you used the O ring, (or if you feel it is too tight, a tiny rubber band) on your cases for the first firing they will be concentric as the rubber will evenly fill the chamber mouth all the way around in front of the rim, keeping the case centred. This is a great help for case life in larger chambers.
The problem of rearward brass flow....
The other huge advantage using the o ring is the base of the case is held back on the face of the bolt which in turn has been slightly loaded against the locking lugs by the action of compressing the o ring. The case does not tend to move forward in the "head gap" for the rim to hit the rear of the barrel when the striker contacts, so there is no rearward brass flow back into the head gap (which, without the oring, would be between the rear of the case and the bolt face) once the case is locked out to the chamber walls on firing. Any ring appearing on the case is just outward expansion of the thin case walls to the chamber in front of the base thickness that does not expand, not rearward brass flow.
Try one in your tight chamber. Then measure the striker indent in the primer and you will see it is closer to the centre of the base than your other first fired cases. Much more concentric. (unless of course your boltway and miss matched bolt are a poor fit or they don't locate properly in the extractor groove. )
I have a case lathe and can accurately test run-out of fired cases. The o ring method produces the best results by far.
BTW, most commercial cases are below milspec size in all dimensions... a lot of the blame for short case life must go to saami, but that is another story altogether...
Subject: Brass life expectancy ?
Posted: 18 November 2013 at 1:41pm
You have missed the point, I'm afraid. Once fire formed (neck sizing only) your cases no longer headspace on the rim, but on the shoulder the same as a rimless case. If you do the right thing by your cases on first firing, then rimmed headspace has no effect after that.
The chamber is a taper cut by a reamer with a set angle. It's diameter dimensions will be proportional to the depth of the shoulder. In a long chamber, the diameter at the base of the cartridge will be greater, and allow greater expansion.
The case sitting in the bottom of a "generous" chamber is a problem. Unless the head gap is .000 or less, the case will not be centred by the bolt face on closing. Once struck by the striker, it is in fact nailed into position, held on the bottom surface of the chamber and only allows brass expansion in one direction- toward the top of the chamber. This is what causes your concentricity issues you are able (due to having a tight-ish chamber, {but obviously not tight enough if the cases are not coming out concentric}) to reduce by rotating your cases 180deg for the second firing.
If you used the O ring, (or if you feel it is too tight, a tiny rubber band) on your cases for the first firing they will be concentric as the rubber will evenly fill the chamber mouth all the way around in front of the rim, keeping the case centred. This is a great help for case life in larger chambers.
The problem of rearward brass flow....
The other huge advantage using the o ring is the base of the case is held back on the face of the bolt which in turn has been slightly loaded against the locking lugs by the action of compressing the o ring. The case does not tend to move forward in the "head gap" for the rim to hit the rear of the barrel when the striker contacts, so there is no rearward brass flow back into the head gap (which, without the oring, would be between the rear of the case and the bolt face) once the case is locked out to the chamber walls on firing. Any ring appearing on the case is just outward expansion of the thin case walls to the chamber in front of the base thickness that does not expand, not rearward brass flow.
Try one in your tight chamber. Then measure the striker indent in the primer and you will see it is closer to the centre of the base than your other first fired cases. Much more concentric. (unless of course your boltway and miss matched bolt are a poor fit or they don't locate properly in the extractor groove. )
I have a case lathe and can accurately test run-out of fired cases. The o ring method produces the best results by far.
BTW, most commercial cases are below milspec size in all dimensions... a lot of the blame for short case life must go to saami, but that is another story altogether...