After a bit of research, I figured out which red dot optic to go with on the ARX-100... chose an Aimpoint PRO. Tried to use my Trijicon MRO w/ an absolute co-witness mount. It does co-witness with the stock BUIS, but like the BUIS the red dot just sits too tall for me to get a proper cheek weld. On the Aimpoint PRO(or CompM4/CompM4s), REMOVE THE SPACER. Now the optic is at the optimal height. Cheek weld issue solved, but now the BUIS are useless. I do not think they are bad, but it seems that Beretta USA does the top picatinny rail/BUIS different than the military version ARX-160. ARX-160 stock sights sit in front and/or behind picatinny rail and slightly lower on the receiver.

Anyone know of a BUIS set that runs around the 1.2" height?