Had two case failures (base of the case) today at the Armscor range using .22 LR's. The first one was with the Walther P22 using Armscor hi-velocity rounds. No damage as far as I could tell both to myself and the pistol. Right after, I fired the Remington 597 using Remington Cyclone (hi-vel HP) rounds. Again a case failure. No damage to myself but I lost the extractor. Will have to procure one (I don't know where to buy them) one way or another.
I think the reason is that the weapons failed to cycle properly and did not lock. Both weapons will still fire even if the bolt is not locked. The bottom half of the casings were probably exposed.
Both weapons have probably gone through 50 rounds apiece without cleaning. This is the first time that this has happened to me. Not sure what precautions I may have to take in the future as regards maintenance.
Dirty chamber probably? Post this over at Rimfire Club, they might be able to help you.