New brass is "on the way". I use fed 205m normally in all my rifles (when I can get it lol).If you want to know the guns potential, why not use new brass and your best powder option while the barrel is low count?
No way to really tell what the cause is since you don’t know how the used bras has been handled. I have shot cci450, br4 and federal 205/m in my Panda ppcs. Never had any problems, even with hotter loads and loser primer pockets that were not too lose.