I'm working up a load for my Swedish RB in 8X58R Danish. Last trip to the range I fired 6 rounds with 43 grains of IMR4895 pushing a 170 soft point jacketed bullet. I had loaded each round carefully, measuring the powder with a scale, etc. 5 of the 6 grouped about 4 inches low. The 6th was 3 inches high (at 50 yds). 5 of the 6 cases had no expansion of the case mouth or shoulder, and of course there was powder soot on the outside of the cases back halfway. The 6th case's mouth was expanded, the shoulder had moved a bit, and the primer was sticking out a few thousandths. Apparently, the pressure in number 6 got a little higher than the other 5, more like it should be.

So, why only one out of the six? Does this happen because I'm still not filling up the case and I'm using a "slower" powder? Should I add a few grains and carefully try again?

I have to add, I don't really know the order I shot these rounds. The higher pressure one could've been the first, last or any in between. I do know I checked the bore between each shot, looking for unburned powder. It was clean each time.

Gunboat, the primer sticking out a little bit is also another one of the very low pressure signs that a cartridge will display. The sequence of events when we pull the trigger on a round is: the firing pin hits the primer and shoves the case forward in the chamber as much as the headspace will allow. The primer then ignites the powder. The powder creates enough pressure that it expands the case to grip the case wall and at the same time slams the case back against the bolt thereby re-seating the primer. I think in your case the case did not slam back and re-seat the primer leaving it somewhat sticking out.

Jn316,
What you said seems to make sense. The other 5 rounds showed no sign of primer protrusion. So, case number 6 stuck "forward" as pressure built up, maybe because of a dirtier chamber or whatever. It ended up sealing better in the neck and shoulder so less gas escaped, higher max pressure, higher velocity....

Seems like I need to add a few more grains of IMR 4895.