Just got a new Marlin 22 mag and the winchester ammo seems to be spliting every 5 to 10 cases. Havn't tried any other loads yet but is this likely to be an ammo issue or a chamber issue?

Semi-auto or bolt? Sounds like an oversized chamber to me....

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MikeG

My brother had the same thing happen using win.38 ammo. He sent Winchester a note and they sent him a voucher for a free box of shells.Q

yep....have a gunsmith cast the chamber

I've had 3 marlins and a 9422m chambered in 22wmr over the last 40 years and have never had an issue like you described with Winchester (or any other) ammo.
I would definitely seek the help of a reputable gunsmith, document his findings, and contact Marlin if there is a problem with the chamber.

Well, down here in NSW, Australia, about 18 months ago we had a couple of complaints about Winchester .22WMR splitting cases. The conclusions we came to were that the rifles may have had a slightly oversize chamber and that the Winchester brass was fairly hard. I did a test at the local range and while the Winchester brass would crack about 20% of the time, PMC ammo was fine. Strangely enough both rifles shot better groups with Winchester ammo than PMC.
Winchester Aust were sympathetic and interested but after that I havn't heard any more. The PMC brass seemed a little softer and perhaps a gnat's whisker thicker than the Winchester brass but I don't have the precise measuring equipment to be absolutely certain but that's how it looked at the time.
Dennis.

Thanks Hilly, I suspect as much and yes the Winchester stuff does crack about 20% and yes it does shoot really well. Will try some other brands and report back soon as I can get to the range again.