the barrel might be ringed or bulged, it usually occurs just ahead of the charged load, as a result of not entirely seating the bullet/sabot on the charge.
It is possible that the barrel bulged but I think it's unlikely
if the projectile still maintains contact. If it fell through the barrel with absolutely no contact then I think it would far more likely.
It's worthy of checking though. Any detectable deformity (bulge or ring) on the outside of the barrel and especially at the "chamber" (I know, there isn't one) would be reason enough for me to make a tomato stake out it.
You could check it with a straight edge on the outside of the barrel and/or remove the breech plug and drive a slug out the breech end to see if there's a noticeable change confined to a small area.
