I smell a potential chamber issue ... a "small" issue can go unnoticed with ammo that is not pushing pressure limits whereas higher pressure loads can have issues. An example, I had a barrel chambered for 260AI several years ago ... fire forming with med-low pressure loads - no problems. When I did load development to come up in pressure (now with fireformed brass) I got intermittently sticky extractions with some hard bold lift. I back things down from the upper published charge weights and still had the same issue. It turned out to be an 'egg shaped' chamber due to what was apparently some runout in the chambering ...
That mark on the shoulder makes me think you may have a burr in the chamber. If so, that would explain a good bit because when you fire it that brass case (pronounced balloon) expands and grabs hold of every imperfection, etc ... it doesn't 'deflate' all the way back (hence the reason we have to resize our brass) and so it could very well be catching on a burr up there. An inexpensive bore scope from Amazon (less than $50 US) may help to find issues. A gunsmith can for sure help to identify if there is a chamber problem.
One other thing ... I'll echo the sentiment of what others have said. Pushing pressure limits isn't smart. If you have primers falling out, you are way over safe pressures. Every one of those 300 over pressure loads has incrementally weakened your barrel and action a bit. Eventually, it will let go and when it does you may not be around to re-read the cautions in this thread.
Regarding the brass on your bolt face - that 'could' be from over pressure ... or ... it could be that by not sizing enough originally, you were in an insufficient headspace scenario such that every time you closed the bolt on a case, if the case was restricted from rotating due to a burr, you were 'polishing' the base of the case with the face of the bolt and subsequently depositing some of the brass from the case onto the bolt face. Insufficient headspace can also lead to wonky/inconsistent or 'spiky' velocities.
If you need a little more speed than you can get with max published loads, perhaps consider having your barrel re-chambered to something 'improved' to gain a little more. This is why I run a 260 Ackley as opposed to a straight 260. +100-125 fps and longer brass life due to less case stretch and far less frequent trimming required ... just a thought. Good luck and be safe.
