It's an odd building for a museum, and there's - mainly - only one route round. Play this to your advantage. Stop where you can and wait for it to get less crowded (at the Sistine Chapel, obviously, but elsewhere too). And if a gallery isn’t open, nil desperandum! - it might not be closed all day (I managed to do three circuits in a day, during which the Bracchio Nuovo, with its Augustus etc, was closed, open, closed respectively!).

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