So I don't hijack the other thread...

I've always wondered about belted cartridges that headspace on the belt:

If you first consider a non-belted bottleneck cartridge that headspaces on the shoulder, proper headspace is necessary to limit stretching of the case. Excessive headspace will result in a condition where longitudinal stretching of the case upon firing will overstress the brass and dramatically increase the chances of case failure and gas leakage. Right?

So, what if you had a belted bottle neck case that headspaced perfectly on the belt, but the shoulder of the chamber is either set too deep, or the cartridge shoulder had been bumped back. In this instance the case would be subjected to the same kind of longitudinal (negative normal?) stress and would be just as likely to rupture, right?

Am I missing something, or is it that by headspacing properly on the belt it ensures that the casehead is completely enclosed, and that any rupture forward of this point would be insignificant because all gasses would blow harmlessly out the end of the barrel?

You'll just end up fire forming the shoulder and neck with little appreciable case stretching. That is because the case head is captured and can't move. Now if you keep re-creating the problem thru incorrect re-sizing, eventually you will get brass fatigue in the shoulder and neck. One side benefit of the belt is that it does indeed create an enclosed 90 degree obstructive path to gas escape. It acts like a fully enclosed rimmed case with a secondary rim for extraction. This eliminates to a great extent the disadvantage of the rimless case while still allowing smooth feeding.

That is why many will headspace a belted case on the shoulder after the first firing.

The mechanics are different with the 2 scenarios. In one case(sorry about that!) the rimless case head will be driven forward too far by the blow of the firing pin while the powder is igniting. The walls of that case, under pressure from the igniting powder, will stick to the chamber walls except at the case head which, you'll remember, has been driven forward. The unsupported head, still under pressure from igniting powder, will then be pushed to the rear of the chamber while the case walls are still stuck to the chamber walls at the front. This causes the case to stretch and the stretch is concentrated in the narrow ring where the case walls start to thicken, down close to the head.

In the other instance, the belt will hold the case head against the breech face and the stretch will occur toward the forward end of the case. The case walls at the front, where the brass is thinner, are better able to expand more evenly over a wider area of the case and so the srretch isn't concentrated in a narrow band. Thus much less tendency to case head separation.

The fly in this ointment is the fact that many manufacturers differ in their belt dimensions. Not only rifle manufacturers but also reamer makers, ammunition makers and reloading component makers. So the bottom line is that you need to VERY CAREFUL when headspacing on the belt. VERY CAREFUL!

Same goes for rimmed cases but their rim thickness seems to be a LOT more standardized across the industry than the belt data.
Regards, Joe

J.D.'s posts have pretty well summed up the mechanics of it.

For hunting rifles, my own personal practice is to consider the first firing of the cases as a fireforming load. Afer that I "headspace on the shoulder", BUT, I set the shoulder back .001" or .0015" when I resize the case. In the field one can never tell when a little crud in the chamber might get pushed forward and become trapped between the shoulder of the case and the shoulder of the chamber. At any rate, a case sized a little shorter than a fired case will almost certainly feed when one needs it to.

I do NOT neck size only for hunting ammo.

And after three firings, I throw the cases away. If I was hunting dangerous game, I would throw the cases away after two firings (one fireforming firing, and one firing in the hunting field).

For target rifle applications with belted cases, I like to buy a specific "lot" of cases and have them in hand before chambering the rifle at all. Then I like to turn the front of the case belts until they are all of uniform thickness. Then I can chamber the rifle just deeply enough for the belt of every case to allow the bolt to close on it.

Then after the first firing ("fireforming"), I try to NOT set the shoulder back any measureable amount. After about 4 firings, I chuck the cases in the garbage, buy another lot of the same brand of cases, and turn the belts to fit again.

The whole schmear is probably totally un-needed and overly complex, as well as expensive, but it gives me peace of mind. Sometimes that is the difference between winning and losing in target shooting.

Not saying anyone else should do any of this and not recommending it. Just telling my own compulsive way of doing it.