I doubt if any exist in the mainstream North American market that said I have an oddball that meets the literal description - but I say again oddball - mounted - that's an Aimpoint 2X with 30mm tube and their adjustable red dot.
Aimpoint makes a couple different sights along those lines - mostly the same internals I suspect but do not know with variations for different mounting schemes.
In this market many of the 30mm tubes exist as more or less add-ons to traditional 1" tube scope designs to allow more vertical adjustment and that's hardly an issue in the 2x-4x range.
Similarly there are of course several high end low power variables but I'm not sure how much advantage a 30mm tube offers in the 2x-4x fixed power range.
