.25 R3M tank dump

My .25 R3M long had been sitting in the closet for a while, so yesterday I got it out and took some shots. At the next to last shot, I thought I heard a funny noise after the shot. On the last shot, the tank dumped through the barrel after the shot. I’m assuming that an o-ring gave way somewhere, and think I recall a recent thread with a similar problem, but have been unable to find it. Any pointers on where I should be looking?