I use a GDI MT6-OSM, which is a lower 1/3rd Aimpoint Micro mount. It is skeletonized so that the middle is empty, and I can use the factory sights off of that. I would think that the 503CU should work given that the middle is also empty, based on pics? That being said, my irons are quite hard to use, since I use a Zhukov stock with a SideProject Link, and the stock sits much more inline than the OEM stock.