Good rule of thumb in a revolver is a blown cylinder / top strap is almost always an over pressure load, a bulged / split barrel is an obstruction. I'd be willing to bet they were firing double action, the squib stuck in the barrel, and a second, normal power load was fired immediately afterwards, causing the failure you note. I have seen it happen several times when we still carried revolvers for duty use, and a lot of guys reloaded their own. Usually when shooting fast double action.
Most likely no powder charge, though it is possible that cases powder charge had been compromised with spray lube? I doubt it was a bad primer, as any primer firing with enough force to push the bullet into the barrel would be hot enough to ignite the powder as well.
Human error happens in any endeavor, including reloading. I have been loading for a long time, and am pretty anal about it, and yet have had a few close calls myself. Best advice is to use this as a learning tool. Glad no one was hurt. S&W should send you a free shipping label to return the gun, though it is iffy they will replace it as they will most likely determine a reloaded round was at fault....But you never know.
Larry
