The 510c is my favorite optic. If I was to put a RDMS on a handgun it would be the 507. The reticle(s) is great, shake awake, nice big sight picture, easy and ridiculously fast target acquisition. I have the 510c on a Beretta 1301T. I've been training with shotguns forever and my times improved dramatically the second I put that optic on there. Nearly impossible to miss. In fact since I put that optic on there I have never missed. The Beretta is unbelievably light and fast swinging. That set up is a total game changer. The solar backup and QD mount made me not even worry about co-witness with the Ghost rings. I could put a Burris Fastfire on it and it would co-witness, but I'm not trading the 501 for that little Fastfire.
I have Romeo 5s on all my ARs and 3 other tactical shotguns, but when I got the extra cash they will be swapped out for the 510c. And in December when I get the M1014 it will get a 510c on it before I even get it to the range the first time.