I have a 12 gauge Model 21 that is somewhat harder to open after firing both barrels. The issue does not occur after firing only one barrel. The top lever moves like it should, so it doesn't seem like a bolt issue. Ammo has been a mix of Winchester and Estate dove loads (not brass bases). The gun is new to me, so I don't have any history to compare to. Is this normal? If not, any ideas what could be causing this?