In reply to: The biggest disadvantage to the O/U is it gets top heavy as it gets bigger, and is a real bitch to get a shell chambered into that bottom chamber when running...at or away!! They have to be opened up sooo darned far, unlike a SXS. A 375 is about as big as I think is practicle in an O/U.
The top heavyness depends much on what depth and style of action is used, what type of stock and forend is used, and also if a scope is fitted.
Big O/U doubles don't have to be top heavy. I've handled a trim little open sighted O/U in 500 Nitro which had no top heavyness at all and ballanced beautifully. Kicked well though!
If you are having trouble loading the bottom chamber while running perhaps you should try a different loading technique. I don't find the O/U style any slower or harder than SxS to load the chambers.