several factors, not enough powder in the case can cause pressure spikes just as much as over charges, incomplete burn or even complete burn early before bullet has cleared the rifle, You already suspect the brass I suspect the charge, The bullets you are comparing are also quite a lot different bearing surface is one, just a thought

sorry I forgot to ask, you fired only one round ?

Last edited by SEM; 09/09/17.
if you want change you have to put in your 2 cents, you can't just sit on the sidelines and whine