I've got an 850 that I bought off a friend of mine for a very good price. The gun has less than 200 rounds through it. The problem is that after any number of shots (10, 12, 20, no real pattern), there will be a very hard trigger pull. Hard enough that you cannot squeeze the trigger. You can relax from the trigger, it doesn't help. If you rotate the cylinder to the next position, the gun will fire normally. Go all the way around, and it will fire on the round that it jammed up on.
If you dry fire, or pull the trigger with empty (already fired) cartridges in it, it doesn't do this. It only seems like it's live rounds that this happens with. Doesn't matter if you're using a slow trigger pull, or a fast one. Has anyone heard of this happening? This is why my buddy sold the gun to me, as he got fed up with it.
ETA: This problem is not specific to one ammo, either. It did it with both Winchester and Remington FMJ rounds.