I have installed many drill & tap bases for the Ruger 77 series over the years. You have to remember that the saddle cut into the base and the integral base on the receiver are both the same width. The problem with these systems is that the Ruger receivers are often warped and twisted around like a pole dancer and the bases often don't fit because of it. I know that some people true up the base sides on the receiver on the mill between centers. But I have always found it better to widen the saddle in the base by putting it in the mill and opening up one side .001 inch and trying it and then opening the other and so on until the base drops on with a friction fit. Always fuck up the least expensive part !