For what it's worth, I run 400s in my 6mm ARC and 450s in 223/5.56.
I have goodly supply of both, but shoot the 223 5-6 times for every 6mm ARC that goes downrange from my ARs..
I ran many of 400s in 556 and proportionately only pierced a very small for percentage. however in 6.5 Grendel and 204 Ruger I have had substantial piercings 450s zero piercings.
As Stick pointed out over pressure is not the only reason piercings can happen. but primer is the easiest and first thing I would personally change and see if it went away.
