[roland]

I don't think it was a double charge, as you said the top cartridge went off while in the magazine as you fired the round in the chamber. I'd say it was a high primer and caught a burr or a spot weld on the mag. And the firing set it off. I'd check the mag for any burrs and file it down. Better yet, crush it and just through it away. ( As it may have been kinked when the cartridge went off.) High primers are a somewhat common problem in reloading. But they ussually prevent the slide from going into battery. I'd also look for tiny cracks along the rails of the frame and the slide just to be sure.