Not sure what caused that but I'd pretty much rule out brass. I've seen magnum brass split the entire length of the case on several occasions during the last 40 years and never had a gun come apart like that. Glad you're ok, that would be a real scare! Talk to Smith and tell them what happened. I'd even ask them if they'd like the gun for examination but you may be right about warranty work(although I don't think they'll hang up!).