Autos are measured like rifles, to the breech face. HK P7 with 105mm barrel should be registered as 107mm, the headspace is 1.65mm or so. Any fraction of a mm is lawfully rounded up, even 0.01mm, which is about half a thousandth of an inch.
Revolvers are being measured to the cylinder face (front of the forcing cone), especially when verifiers are officially equipped with a simple rod. Most use digital calipers, which is what CFC will insist on seeing photos of. Recently some CFC staff have insisted on measurements from several calipers. All nonsense, of course.
S&W 66 can definitely have a 105mm barrel, and with the cylinder gap, this becomes 106mm, rounded up. They are certainly worth measuring if you own a prohibited one.
I agree with the rant, but we all heard it, no changes to laws about Prohibs. Those laws are keeping us all safe. So measure away, you have better odds than buying lottery tickets.