You don't need a rail, although it makes it easier. You can shim the rear mount about 10 thousanths of an inch to 15 thousanths of an inch to get on target. I went through this with Nikon, and found better information on the web than Nikon could offer. If you are using standard mounts, and a long range scope
, you will probably need to shim the rear mount about 10-15 thousanths of an inch.
Rail mounts are easier because mounts with the 10-15 thousanths of an inch cant are readily available.
My rifle was hitting 6 inches low with the elevation adjusted as high as it would go. After shimming the rear mount 10 thousanths of an inch I could hit dead center at 100 yards with the elevation adjustments approximately in the ceter of the adjustment range. Now I am getting 3/8" - 3 shot groups at 100 yards.