I wrote this up a few times in detail, and it's gets into some pressure details. But basically, you are using an action that was designed for large primer rounds, and changing to a small primer case. But the firing pin is a large firing pin in a standard action. When you switch to a small primer brass, you are using a large firing pin to strike a small primer brass. I'm not 100% certain of the sizes of the small vs. large firing pin, but I think the small is .060" or .065" and the large firing pin is .075" or .080". Either way, you are striking a small primer with a large firing pin that is creating a dimple on the small primer that is about 33% larger than a small firing pin. This dimpled area was designed for a small firing pin, and in comparison to the surface area from a small rifle primer to a large rifle primer area, is HUGE. So, you are creating a weak spot on the small rifle primer with a large firing pin. If you are using a thinned cup primer (see picture below), like a CCI #400 or Federal 200 primer, the cups are very thin and can easily (and most likely) cause pierced primers, even on light or mild pressure loads. I run a CCI #450 primer in my 6.5x47 and I was getting pierced primers with my Surgeon action, which is designed for large rifle primers. I sent my bolt off to Greg at Gre-Tan, and he did a great job "bushing my bolt" down to a small rifle primer firing pin. If you are experiencing pierced primers on mild loads, switch to a thicker cup primer, like a CCI #450 or CCI #41 primer. But, it may not 100% solve your issue, like it didn't for me. You may need to send your bolt off to be converted to small firing pin. Once you do this, it will solve your pierced primers, and there is not negative shooting large primer rounds with a small firing pin either.