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Oh, about .1811024 inches, as the crow flies. But seriously, folks, in the last few years scope makers have introduced wider 30mm tubes. The reasoning behind such actions is that if a larger objective gathers more light, a larger tube should be a good idea, too.
The main consideration in a 30mm tube scope is that you will need 30mm rings. Swift rifle scopes are available in both 1" and 30mm tubes.