ARMS #38 rail with sight. I like it, but it is not adjustable for elevation, nor can it co-witness with an EOTec. If co-witnessing is not a priority, then the #38 is very nice. However, I wanted it to co-witness. The ARMS#40 I like a lot better but it also is not adjustable for elevation. No matter, the front sight is. And, the #40 will co-witness with no trouble. I do not like the factory rear aperture with the "horizontal" line on the long-range aperture; with my eyes, the aperture gets lost with the top half of the sight missing. While the #40 is available with the standard A2 aperture, #40A, the #38 is not so far as I am aware. On both the 38 and 40 I switched them over to the standard A2 aperture which only takes a minute. I also like the way the fold down out of the way and are spring loaded so they cannot be folded down accidentally yet spring up immediately.