Here's a little secret one should really know if you intend to mount a scope on a Mosin... Those scope adapters that drop into the rear sight lower to replace the leaf and slide are junk. BUT, the rear sight assembly is mounted on an 11mm Dovetail. Knock out the two lower retaining pin first. Then you'll need to heat the sight assembly with a propane torch carefully if you intend to salvage the original sight (they were usually soldered to prevent any movement, so you have to "un-stick" it), then tap the entire rear sight assembly off with a wood block. What's left is an 11mm dovetail to which you can mount a scope base and rings for a LER scope that is true to the barrel, solid, AND the right height. When finished, it looks something like this:
MUCH more solid than the drop-in style, and it snaps up with the correct cheek-to-stock rather than being an extra inch too high for comfort. When I mounted this scope, I pulled the bolt to get a rough bore-sight on a stump about 200 yards off from my back porch only to find it needed no adjustment--dead on with the bore. Took me about 5 clicks of elevation to tighten my 100 yard zeroing after mounting. If you're mounting a LER scope on a Mosin, there IS no other way to do it right.