M96 6.5x55 problem with 160 grain loads

fourfive8

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Yes, that is not good! Looks like either really high pressure or excessive headspace. A headspace comparator gauge that compares the headspace length between a “suspect” unfired round and a fired round that has conformed to the chamber will tell that story. A .001” or so difference in bullet diameter with a conventional cup and core bullet wouldn’t likely cause that much pressure increase with an otherwise reasonable load.

Also be sure to trim case length— a long case will crimp the case mouth into the bullet at the end of the chamber which can really cause high pressure. A long case can usually be felt as stiffness right at end of bolt turn during chambering.

I think all the possibilities as others have posted have been brought up in this thread. Probably worth a check off process for each possible variable to eliminate the most likely. Failing IDing the culprit, definitely time for visit to gunsmith.

A basic headspace comparator gauge is very simple and can easily be made. Find some thick wall tubing or bushing (aluminum is easy to work with). Needs an ID a few thousands larger than the cartridge neck diameter and slightly longer than neck. Ends need to be square cut (important). Slip the gauge over neck until it contacts shoulder... should be just outside neck junction (approximate location of datum ring). Measure length from end of gauge to base of cartridge. Compare a cartridge fully fireformed in chamber to resized cartridges.
 
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