I don't know his hunting habits, but I dont think he'd be that careless
It's probably a combination of factors. Some of the shotgun/pistol powders are very similar. (iirc) 700x is black flakes, and Win 231 is gray granules. Now to those of us who use black powder all the time, we can see these at a glance, but a relative novice?
Then you have the fact that they use the same powder containers to package the powder, simply changing the labels. So say one was using a BP substitute such as Pyrodex Select or Aliant Black MZ, which comes in the same black plastic container and has a black label just as IMR 800x or Winchester AA.
Not only is the type of damage consistent with smokeless loading..., I'll bet either the shooter confused a can of 800X with a BP substitute or the clerk at the shop where he bought it did so.
LD