Just to add to the single aspect of lock-up, there was an article in a gun magazine, I wish I could remember which one, on shotgun lock-up. It seems that during firing the forces are "normally" back and up so the tendency is, because the hinge pin being the fulcrum, to keep the gun closed. Very little force is needed to keep the gun closed during firing but the hinge pin takes a beating (perhaps that would explain those worn hinge pins). In fact there was a picture where the author kept the gun closed with thumb & fore finger. In essence the lock-up, keeps the shells from falling out and multiple locking systems look cool and sell but as 2-p states very difficult to have all points make contact. --- John Can.
