I saw a used MP15 at a gunshop today. The gas block was machined for a backup sight, and on the end of the block there was a knurled screw that has two potential positions on the block. I could not move it by hand, and it also had a screw driver slot, so it may have needed loosed before moving it's position. Anyway, what does it do? Here in PA we can't hunt with a semi auto, but if it was a cutoff, I guess a bit of epoxy would keep it legal in a manual operated mode. I could not find that detail on the SW web page.
