I loaded up 100 plinkers for my Chinese Type 53 , assorted brass, large magnum pistol primers ,6 gr of longshot over a Lee 312-185-2r. These were made of mixed wheel weight, watercooled ,gas checked, homemade lube, heavy on the beeswax, and measure .315. Shooting along at a nice slow pace , grouping 1-1/2 inches at 50 yards- best so far- then it got me- gas around the bolt. Three rounds out of 100 did this, all the same brand of brass. All have a nice sooty neck on them, should I have fired a more powerful charge to form the brass first before plinking with it? The Mosin vents straight back in my face so I would really like to avoid this in the future. What did I do wrong.....