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I was just testing .357 mag loads this past weekend. I had some accurate #9 sitting on the shelf and I wanted to use it up. I had plenty of odd brass and some Winchester magnum primers. I used the starting load, 11.2 grains under a Hornady 158 gr LSWC, according to loading data:
http://www.accuratepowder.com/wp-conten ... eb-REV.pdf
which lists Winchester magnum primers.

The primers were flat after firing. I chronographed the load out of my 686+ with a three inch barrel and got 1145 f/s with a SD of 21.81. I did not expect to get flattened primers with the minimum load. The load data shows 1104 out of a six inch barrel.

I loaded up the same round, but with CCI standard primers. I got 1110 f/s with a standard deviation of 17.26. The edges of the primers were rounded, not flattened.

I am puzzled as to why an increase of 35 f/s would require so much more pressure that it flattened the primers. I suspect the Winchester primers are made of softer alloy.