I had same problem with same gun (El Patron). Sent back to Uberti, they returned it, still had same issue. Cylinder failing to lock up, probably a timing or bolt not seating properly in the cylinder. Only happened about every 10 to 15 times and only when attempting to fire it fast. Even the gunsmith at the shop I bought it at tested and agreed it was defective, but would not fix it as it was under warranty and Uberti wouldn't fix it either. Traded it back in for a Cattleman that didn't do that.
Added: eventually it will stop close enough to set off a round that is not aligned with the barrel and spray lead back at you. Not good.