Here is the data:
Rifle: Mauser action 6.5x55
Brass: New Remington
Primer: WLR
Powder: IMR4350, Reloader19
Bullet: 130gr Nosler Accubond
Loads direct from Nosler:IMR4350
Starting 39.5gr, Max 43.5gr
Reloader19
Starting 43.5gr, Max 47.5gr
Went to the range, shot some PMC factory 139gr to zero the scope, no problem. Next, shot 4 round group of reloads, RL19 at 45gr, again no problem. Next, 1st shot of RL19 at 45.5gr, blown primer. Stopped with the RL19 at this point. Next I tried the IMR4350 at 40.5gr, blown primer. Stopped for the day. Got back home pulled bullets, remeasured powder charges, were ok. Seating depth is .06 off the lands. Not sure what else to check, headspace? Help. Thanks

Is that a Swede? I had a blown primer with 46 gr's of 4831 and the 129 gr Hornady. I had to beat the bolt open. My friend had an SEE event with his too, same load. My rifle has minimun headspace because I changed the barrel when I customized it.
Seems to be common with this caliber and slow powders. I changed to Varget and gained the same accuracy of 1/2" at 100 yd's. I don't know if it is because of the long throat or not.
I would go to a faster powder and see what it does.

Yes it is a swede. I had a heck of a time opening the bolt with the IMR 4350.

Just because the book says its safe it doesnt mean it is in your rifle.

Nosler may have used different brass with a larger internal capacity and a test rifle with a larger chamber. This will reduce pressure.

Its the old story, start with starting load and work up.

Check that when resizing you arent setting the shoulder back and therefore creating excessive headspace.

Back off your sizing die and then start tightening it and checking case until it is a nice fit in chamber.

If you Chrono your loads you may find you are getting equal or better velocities to the max loads in the manual with less powder.

Pete and I only neck size. The 46 gr load of 4831 is a light load. I stopped it from happening by going up to 47 gr's. However the accuracy was not as good. It was strictly an SEE event, not over pressure and velocities were low.
Since no one knows how it happens, I can only guess at it. It is always caused by reduced loads of slow powder. With the long throat of the Swede, there is no resistance when the bullet exits, in effect, the case gets larger. (One case might have a little less neck tension. Maybe a good crimp would prevent it too.) Then the bullet might stop when it hits the rifling causing a blocked bore effect. Who knows? I don't.
It doesn't happen in rifles like the Weatherby's with freebore because they use hot loads of slow powders, it will if loads are reduced below what is listed.
The old Mausers have weak actions compared to modern rifles and all recommended slow powder loads are too light for the case but deemed safe for pressures.
My solution was to go to a faster powder, Varget, and have had no problems since. You can almost bet that the factory loads had a faster powder in them.
It happens once, BAD, it happens twice, shame on you!
I can't argue the point but will never load a slow powder in the old Swede again.
A hotter slow powder load in the 6.5 using a modern rifle would be safe.
If you notice, his 4350 load was 2.5 gr's under the listed minimum and the RX 19 load was just above the minimum. That scares me more then using the max load.