Actually the set back occurs on the top lug abutment. The bottom lug abutment is has less support from the mag opening, so it flexes with the bolt thrust and transfers most of the pressure to the top lug abutment. The mag well opening is set .050" forward of a standard caliber version to account for the longer magazine, leaving less material to support the lug abutment. I've seen some ultra mags that not only had set back on the lug abutments, but also compression on the top bolt lug.