Neither an Engineer or Professional Smith but I have only one word for carrying a gun around open & that's "DON"T".
As Stan said you're putting undue stress on the lug & as in the case of some guns if it doesn't have a good barrel check you put a lot of stress on the forend lug. This is more apt to be the case with a hammer gun than hammerless but can occur with either. Some hammer guns in particular will open until the forend iron contacts the bottom of the frame. When carrying such a gun open there is constant pressure of the forend pushing forward on it's lug. I have an old H Pieper 12ga hammer with a loose forend lug which most likely came about from either constantly opening it too hard or carrying it about open, can't really say which as it was like this when I got it.
As to safety, the primary concern regarding safety, assuming a responsible person to begin with, is the "UnExpected". What's to say that when/if you fall or whatever the gun is not going to snap shut. In this case you then have a closed gun, hammers cocked & exposed to being struck & no safety to help prevent the triggers being tripped if something shout hit them.
Personally I feel a hammerless is the safest, a rebounding hammer next & non-rebounder last. I have hunted with all three types without shooting myself or anyone else so far. Hopefully that will continue.
