I have been loading 82gr of h1000 or 88gr of 50bmg in my 257stw with 120's for several years now and never have had any issues the othermorning when i was at the range it was in the mid to high 80's around 8 in the morning. Well i noticed this for the first time that the firing pin was either cutting a small hole in the primer cup that looked like a punch was used perfectly uniform in diameter, the other thing i noticed was that a deep pin mark was left with a high burr left. The bolt was not sticky nor was the primer flattened or the pocket deformed in anyway. Could this be that my firing pin is to deep? Or is this a sign of excessive pressure?
Are you baking the rounds in the chamber for more than 15 seconds before you pull the trigger.
If the barrel/chamber is warm/hot and you chamber a round and let it absorb heat then it is just like shooting in 120 degree weather. In fact a "cool " barrel will absorb enough sunlight to bake a round and get pressures high.
If you keep your head and your bullets out of the sun you won't get melanoma nor high pressures. Bullets with melanoma are not very accurate. Brains with high pressure get strokes. Or perhaps I have that backwards.
ok ok bust the new guys chops, anyway this was done early in the morning because texas heat is like hell heat at 10a.m. and the range i shoot at has a large awning which is made of aluminum roofing anyway overhang is about 7feet front and back and the benches are centered on a ten foot slab they were built to minimize exposing cartridges to the sun, good thinking eh? I shoot bench rifles and get the gun settled wait for the barrel to cool and then insert cartridge get resettled and shoot fast as to not "bake" the cartridge in the chamber, just cusrious as to what the cause may be I backed the shroud and pin out 1 turn and have somewhat eliminated the problem, i have just never seen this without cratering or some kind of deformation.
even though you have a very overbore cartridge, which is susceptible to pressure spikes, i don't think pressure is the problem. I'm a firm believer that the only sure sign of pressure is a sticky bolt. if your primers are priercing and you don't have a sticky bolt, my guess is the firing pin spring is getting weak.