separating cases

A friend of mine has a really nice old FN Mauser in .375 H+H. It still nips on the go gauge. He has been reloading for years with no problems but is recently having case separations in the usual place just ahead of the belt. 2-3 reloads from a case and there it goes.
Any ideas as to what causes this, other than sloppy headspace?
I have suggested that he adjusts his sizer die so it headspaces on the belt and the shoulder. Would this help?
Additional ideas would be appreciated.

He should adjust the sizing die so that the cases headspace on the shoulder only. Otherwise he is setting back the shoulder with each reload and stretching the case with each firing.

The belted magnums headspace on the belt, that is its purpose. If your friend is resizing and setting the shoulder back with each reload, the case will stretch and separate at the belt. He should try just neck resizing and not setting the shoulder back at all. In a good bolt action ar single shot rifle, if cases are to be reused in the same firearm, there is no real need to full length resize with each reloading.

precision tools said:

. In a good bolt action ar single shot rifle, if cases are to be reused in the same firearm, there is no real need to full length resize with each reloading.

Click to expand...

What you say is true however, if this particular .375 is used on dangerous game, most hunters will opt for full length sizing to avoid feeding issues during "oh shit" moments.
I've recommended at least some full length sizing to clients with the mid to larger bore calibres in guns where reliable feeding is a priority.

If used on dangerous game, questionable brass and/or reloads should not be in play.

If, however, the aim is to get the most life out of the brass, all steps should be taken to avoid overworking it. It might be well to take a chamber cast to determine if the chamber is to spec. Since the round headspaces on the belt, headspace gages will not give a complete picture of what is going on and the distance to the shoulder could be over length causing excessive stretching.

For a quick check..

Mic fired case right in front of belt.

Compare to new brass..

Newer production brass might have a slighty thinner wall at failure point. His previous brass might have been thicker.

Chamber might be oversize at rear, due to non floating reamer holder etc...

Just like many of the old .303 British SMLE chambers, headspace was OK, but chambers were cut large enough to fit a bale of hay alongside cartridge, and still shoot...

Since the brass case starts to thicken internally at base, brass stretches/bulges at first area of thinning.. Roughly belt area on a Mag case... Fails at bulge..

I have neighbor with a FAL semi... The factory chamber is large enough at rear, that Factory Remington .308 brass, does not survive even one firing..

Military brass is thicker at base area. Not much .375 military brass though..

For working up a precision load for that rifle, neck size only if the brass is used in only that chamber. Dangerous game? Once fired brass reloaded for that rifle. Measuring the case volume is a must when creating precision ammo, one batch of the same brand can have larger or smaller volumes and that will affect chamber pressure, velocity, etc...