Hope someone can shed some light for me.

I bought a 1917 M96 Carl Gustav 6.5 x 55 with a number 3 bore and, unfortunately, unmatching serial numbers. As it came completely slathered in Cosmoline I assume it's an arsenal job "franken-rifle" that was cobbled from parts for reserve use.

I ran 2 boxes of Winchester factory 140gr through it with pleasantly acceptable accuracy. However, after I neck-sized them to reload, I noticed cracks in the shoulders on approximately 25% of the cases.

Most of the ones that cracked had multiple cracks around the shoulder instead of a single split - some with as many as 4 separate cracks.

Case expansion does not appear to be excessive - I only neck-size 3/4 of the neck and the "step" is barely detectable. Any clues? Do you think this is a headspace issue (I don't), sloppy chamber tolerances (I hope not) or a defectively lot of extra-brittle brass (I hope so!)

Thanks

Brittle brass I have shot guns with excessive head space they normaly leave a ring just for ward of the base. I would say this is bad brass.

I might be inclined to blame the brass although certainly it won't hurt to have a gunsmith check out your headspace. I have 3 older M96 Swedish mausers in 6.5x55 and have never experience this problem. I can only assume you're not loading them up too hot for your rifle to handle. Best wishes.

Cal - Montreal