I have a 517, 577, and and EXPS2 (OPMOD). I prefer the Eotech reticle lower third co-witness, so the 7mm riser base works fine for me, I prefer it for the less-critical cheek weld. The OPMOD has the QD mount and I've found it to be well designed and solid with an excellent repeatable zero. I run the 577 on a beater rifle with a 3X FTS magnifier. That thing has been abused horribly but has been utterly reliable.