Holosun Technologies ARO and HM3X Magnifier, Red Dot, 2 MOA Dot, Fits 1913 Picatinny Rail, 1.63 Lower 1/3 Co-Witness Mount, Black ARO-RD2-HM3X ARO (Advanced Rifle Optic) is an upgrade from Holosun's popular 20mm 403 and 503 series optics. ARO features a new 22mm housing design that fits perfectly with their magnifier series and a 1.63-inch lower 1/3 co-witness pedestal mount. ARO is available as a 2MOA dot only or with their Multi-Reticle System (MRS) in Red, Green, or Gold.