Is there a standard for the barrel plug sizes for AC adapters/wall warts?

I have a surplus-to-requirements car reversing camera that I've decided will make a pretty cool in-wall laparoscope when attached to a coat hanger. I've got an AC adaptor in my collection that is the right voltage and has the right barrel size to drive either the camera or monitor but not both, so I need to get another one. I can measure the outside of the barrel with calipers, but I'll be jiggered if I can measure the inside. Is there a standard or reference for these so I make sure I get the right thing?