Bushmaster modified the AR design to work with a center-feeding magazine (FAL) as opposed to an alternating-side magazine (standard AR, M14, AK). They accomplished this by removing the 6 o'clock lug from the barrel extension and turning it into a quasi-feed-ramp. Some rifles are living long, productive lives (such as TIMC's above). Many more have not, though, suffering lug breakage due to less-even distribution of forces.
A simple design change of rotating all the lugs 22.5 degrees so that you keep all eight of them while trading two feedramps for one would have worked better, in my opinion. The barrel extension and bolt are .308-specific, anyway, so there's less of a need to satisfy cross-compatibility with other manufacturers.
In short, it was a neat idea, but could probably have been executed a little better.