Can anyone suggest why I am getting this problem.
I have a second hand Sako 75 in 6.5x55 which has performed flawlessly to date. A couple of weeks ago when out stalking I picked up a spent case and noticed it was covered in black soot. Later I fired two more which also came out black and one had pushed the primer out and had pressure marks on the back of the case. I stopped shooting and didn't fire any more rounds that day. The rounds I had were all from a batch that had been fine up till that point.
I cleaned the rifle once home and tried some more rounds only for it to happen again. I then changed the rounds to a different load from 120 grain projectile to both 85 and 129 grain projectiles, neither of these produced black cases. So I swapped to use the 129 grain loads and had no problem next time I went out. Since then while zeroing the 85 grain loads have started producing black cases and then yesterday when stalking the 129 grain loads did also but only on the first shot which was through a clean barrel then it didn't do it again firing another three shots that day.
I've asked several people about this and nothing seems to make sense. The loads are all no where near hot loads, they all use different powders and I've had no other pressure signs like flattened primers or stiff bolt lift. Any help would be really appreciated as I'm starting to get really pissed off with it.