One thing I have had to do on a couple of rifles was to make room for the front scope bell.
I seem to have to mount my scopes rather far to the rear and sometimes the front bell is actually over the rail.
On one rifle, I cut the front of the rail to make clearance, on another, I flattened the rail by grinding/sanding off a few of the picatinny "bumps" on the top of the rail.