The CR2 in PR used to have something that was similar to this, but it was changed for some reason.
This is what the sights look like in A.C.E, and should look like in PR too. Notice that I'm zoomed out here, and that it's equilient to being zoomed in in PR. And of course that the optics is placed on the tip of the barrel, not at the bottom of it. It makes it much easier for the gunner to determine if the terrain, trees, etc are blocking the cannon.
(click on the thumbnails for full size)

Screenshot of the gunner sight in Steel Beasts (the first game):






