Wow, bummer. Did any of the rounds "feel" different? My guess is ammo had nothing to do with it and it was a manufacturing defect. I hope S&W sees it that way and you get a new gun. It may just be light reflecting off unburned powder but what is the yellow stuff that looks like corn meal?

The Model 29 is a pretty stout gun, but even the best gun manufacturers drop a t**d now and then.