One factor that you left out of your original post is where you were shooting. I been hit by jacket fragment at my local indoor range several times when I was loading my gun. They were ricochets from the bullet traps from other shooters. My lane uses pyramidal bullet traps for each lane and an "edge strike" will throw debris back to the shooting stands.
I've also caught jacket fragment in the face twice when shooting full power 357 Magnums with my 620. Close examination of the forcing cone, a careful timing check, and a re-check of the B/C gap revealed no issues with the gun. B/C gap check between a tight 0.005 inch to a just barely loose 0.005 inch depending on cylinder position. Checking the forcing cone immediately after the first occurance showed no evidence of a shaving issue and every cylinder position was checked for alignment with a gage.
The final choice for the cause was that the bullet traps needed cleaning and my Magnums had enough energy to kick debris from the trap to the firing line in spite of a center hit on the trap.
However, in my cases of being hit by fragments, at no time did they draw blood. They just stung a bit and reinforced the need to never do any shooting without protective eyeware.
Point of all this is simple. If you were using an indoor range, don't make the assumption that you were hit by fragments that you fired. They could have come from a shooter a couple of lanes over. They could have been from your own trap that may have needed to be cleaned out. So, don't panic just yet. Have the gun looked over by a qualified gunsmith locally or do it yourself if you know what to look for. Also, make sure you don't clean the gun after an incident like this, if your shaving brass a thorough cleaning will remove any evidence of that. If you, or your gunsmith, see an issue, then send the gun back to S&W for correction.