I have an active post on the forum, on reloading the 25acp, and I am experiencing, something I don't completely understand. A different topic on the same subject. I have experienced cratered primers in some on my top test loadings. How does a .357 mag, loaded to 35K pressure, not have cratered primers, while a 25 auto, loaded to 20k, have them, both using the same brand of SP primers? Is it the blowback semi auto function, or something else? Maybe an oversized firing pin hole? Should I ignore this, and go on case head expansion measurement instead?
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compare flash hole diameters. A larger dia. lets more pressure on the primer cup.
The other common issue is a small diameter firing pin working in a large diameter opening in the breech block. Especially if the firing pin protrusion is excessive.
My remington 700 223 will show a slight crater even at low pressure cast loads; with jacketed full charge loads it gets more pronounced. Examining the bolt face with a jeweler's loupe shows that there is a very slight radius around the firing pin hole. In my experience, primers will make a mirror image of the breech block or bolt face around the firing pin even at moderate pressure. Take a look at primers fired in a Glock. Also, and somebody please correct me if I'm wrong here, but I don't think you would see reliable case head expansion until you were past a safe pressure in your 25 acp.
It could also be a problem with the primers themselves. It has happened before but is very rare. Back in the 80's, I believe, I was shooting CB benchrest using a pair of Stolle Action guns. All of a sudden I started seeing pock marks on the boltface around the firing pin hole. It turns out that the primer manufacturer had bad metal in the primer cups and they would leak around the edges with enough gas to cut the bolt face. A lot of good bolts got damaged before it was realized that the primer cups were at fault. The company replaced any primers from that lot that were left free of charge.
You might want to try some other brands of primers to see if the problem persists. The normal hardness of the cups does vary from one brand to another. Also check the end of your firing pin. It should have a rounded end on it with no chips or damage and no sharp point. Normally, firing pin protrusion should be in the range of .050" to .060". If it sticks out more than that, that could be the problem.
Most of us old timers here will remember that long ago problem!!!
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