Are magnum primer cups thicker or harder than standard primers?

Z

zanemoseley

New member
I need to get back to shooting my 6mm Fat Rat AR. I found that 30.0 grains of RL15 under a 105 grain Berger Hybrid Target was very accurate, I shot a 196/6x on a 600 yard F class target.

Problem is when I was load developing I had no primer issues. But then I loaded 60 to shoot some F class targets and wasn't paying the best of attention and had a bunch of pierced primers when I got home and was decapping. I was using the same primers that I was when I was developing the load which were standard CCI standard rifle primers. One guy I was comparing reloading notes with on the 6mm Fat Rat was using magnum primers. Are they tougher than my standard primers? I intend to drop down to at least 29.8 grains of powder but I'm wondering if I should swap to magnum primers too.
 
As a general rule, magnum primers are a bit thicker and hotter igniting a bit more powder initially and slightly spiking the pressure higher than a standard primer would in the same load. If you want to switch, and you are working near the max, you should drop you powder charge a bit and work back up. Otherwise you may wind up with stuck cases or very heavy extraction.
The other thing is, if you have a light pull competition trigger, switching to magnum primers can lead to FTF's . But that's really only important if you are down around 1 lb or so of pull.
Also, some powders are more consistent with magnum primers and some with standard. Flake and stick powders seem to me more consistent with standard primers and ball powders with magnum.
 
Yes. That old table of Calhoun's shows large rifle primers all use the same cups, but small rifle primers do vary, with magnum primers being thicker. Assuming yours is based on 6.5 Grendel brass, you have small primer pockets and will see the difference.

With pierced primers you also want to confirm your firing pin protrusion and tip shaping are within spec. If you were shooting in higher temperatures than you had when you developed the load or let sunlight heat the cartridges before you used them, or let them sit in a warm chamber longer during the match, these will also raise pressure, more so with some powders than with others. RL15, specifically, was dropped from the Mk. 316 mod 0 sniper round development due to pressures getting too high in the heat. Federal went back to 4064 for that round because of it. You might try the same thing or Varget as less temperature-sensitive alternatives.
 
Unclenick said:
Yes. That old table of Calhoun's shows large rifle primers all use the same cups, but small rifle primers do vary, with magnum primers being thicker. Assuming yours is based on 6.5 Grendel brass, you have small primer pockets and will see the difference.

With pierced primers you also want to confirm your firing pin protrusion and tip shaping are within spec. If you were shooting in higher temperatures than you had when you developed the load or let sunlight heat the cartridges before you used them, or let them sit in a warm chamber longer during the match, these will also raise pressure, more so with some powders than with others. RL15, specifically, was dropped from the Mk. 316 mod 0 sniper round development due to pressures getting too high in the heat. Federal went back to 4064 for that round because of it. You might try the same thing or Varget as less temperature-sensitive alternatives.
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Thanks that Is helpful. I don't have temps recorded but I was developing the load in March and April, got busy and didn't shoot for a while, the day I had all the pierced primers was July 3rd so definitely hotter and I was pushing the limits a bit so I could see temperature being a factor. I guess this is my first lesson in temperature sensitive powder.

I did get good results from the RL15 so I'll drop the charge to see what I can do. I did try Varget and seems the accuracy wasn't quite as good as the RL15.

I still need to develop my mag length load so perhaps I'll look at Varget some for that.
 
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