The plot thickens..
I had cleaned the rifle, done a little polishing and then of to range for test firing.
First 50 rnds (2 mags) seemed to be discharged without any problems, but then the problems started. Around 20% of the rounds failed to ignite on first try, but most ignited on second try, after being put back to magazine and re-cycled into the gun.
I collected some examples of fired casings and also some non-ignited rounds to check the firing pin markings. Then I also noticed that the first 50 rounds were not so successful and had included some bulged and deformed casings.