The gun is in 7.65x53. Obscure variations may be interesting but they are not related to my problem.

Why would the case stretch and stretch in a way that trimming or full length resizing can not fix? Could it be a bad die? Is there any chance that this is a headspacing problem?

Any ideas?

The original question, listed below, was why would a Mauser M1891 only get three loads to a case. I always full length resize. I've tried trimming the brass. I've switched bullets and powders. I've tried formed cases from Buffalo Arms. I've tried new Hornady brass. Nothing helps.

The first time a case is used it works perfectly. The second time there is some slight resistance. The third time chambering is HARD. You'd have to close the bolt with a hammer to get the fourth loading chambered.

These are NOT powerful loads. Fact is that the last powder I tried was 4 grains below the minimum listed in the Accurate Reloading Manual.

Sounds suspiciously like progressive neck width increase, through brass flowing to the front more and more. Have you miked the case necks of the loaded rounds through the various reloads ? Have you turned the necks ?

Regards,
Carcano

How much are the necks stretching?

Are you FL sizing down to the shell holder?

Are you using the right shell holder?

Feel inside a case with a wire. Are the webs thinning?

You need to determine why it's getting progressively harder to chamber the round, could be that the cases are too long due to stretching, you can tell if this is the case by measuring them and can fix it by trimming them. You said you trimmed, so that shouldn't be it.
Are you sure your dies are properly adjusted? Maybe you aren't really full-length sizing if they aren't properly adjusted.
If the case necks are getting thicker due to brass flowing, you can determine that by measuring the neck thickness, or if you don't have the tooling for that, seat a bullet and measure the neck diameter with your mike. If they are too thick you can either ream the inside of the cases or neck turn the outside of them. You didn't mention that, so I'd check it out.
Old military rifles often have headspace issues and that's something you might want to check, but generally they have excess headspace and that wouldn't usually result in hard chambering, hard extraction yes, but not typically hard chambering.
Are you using cases formed from longer cases such as the 30-06? If you are, I'd take a real close look at the neck thickness issue.

good luck and regards,
Graycg

KLW I think your problem is related to an improperly adjusted sizing die. You may have exceessive headspace that results in case stretching. When you full length size the case you introduce more headspace. Try taking 2 or 3 new cases ,load them and fire them.When you size them DO NOT full length size the cases. Back your size die out of you press about 2 full turns so that you only resize about 1/2 the neck. This case should still fit your chamber perfectly and will not stretch anymore. Load it and fire it again and again. Make sure you keep these cases separate. I think you will find your problem disappears.