Ken: glad you're ok. If possible, check the top two rounds in the magazine to see how much 'blow-by' or powder burn they have on them and how it patterns. If it patterned toward the top back half of the case I would blame the ammo. If it patterend on most all of the top case in the mag and blew some residue down onto the 2nd case in the magazine I would say the firing pin may have touched the round off before the slide chambered it fully.
But, if your firing pin functions properly (retracts under spring pressure) and the channel was clear, the only other problem I can think of is that there may have been some primer material from the preceeding case that jammed the firing pin forward so it acted like a fixed firing pin that detonated the case before it was fully chambered, but then retracted properly back after the round detonated. If you can check the preceeding spent cases for primer flowing indicators (that the primers were too soft or the rounds too hot) that might help.
Was the blown case blown out at the bottom, at the unsupported part of the barrel/chamber? If so, it was the ammo.
You said it 'appears the slide was not fully closed' - why do you say that?? If this was factory ammo there should not have been a chambering problem and unless your firing pin was jammed full out the round should not have detonated out of the chamber - it should have slam fired when the slide closed. You did not say it fired when the slide ran into battery so it must have fired when you dropped the hammer - bad ammo. Bad Karma but better luck for you that you were not seriously hurt.
Get your $ back from Winchester or the dealer and wear protective glasses next time you shoot. HTH.
Oh yeah, and btw - disconnectors should have a flat spot on their heads.....