By Mark V. Lonsdale, STTU Training Director
While running the return to zero evaluations in the previous article, we decided to try moving the mount further up the rail to see if it made a difference. We were using an American Defense UIC rifle with an AD cantilevered mount and a Leupold scope, so already knew that we were using high quality products that consistently returned to zero when removed and replaced. But when we moved the mount forward on the rail, the result was a dramatic shift in point of impact.
In the image above you can see the placement of the scope mount at the forward edge of the received. The cantilever overhangs the fore-end handguard, but the actual mount is on the receiver. This is the same index reference we used for all the previous tests.
When the mount was moved one slot forward on the Picatinny rail (see arrow), the grouping at 100 yards moved 10 inches to the left. When moved back to the last slot on the receiver the group returned to zero. The reason for this is that the rail on the handguard is not integral to the receiver, so not machined in one monolithic piece. So while the rails appear to be in line, there is sufficient offset to throw the shots 10 inches to the left at 100 yards.
Lesson Learned: Always take note of exactly where your rings or one-piece mount are indexed on the receiver rail. Count the number of slots from the front or rear so that you can always reset the mounts to the same location.
Leupold Mark 4 mounted on an American Defense UIC Mod 2 .223 Wylde