I'm a fan of iron sights on pistols, but if I was going to put an optic on one, it would be a "See All Open Sight". These are available from Questar, which is where I got mine. I use mine on my 10/22, but they work well on almost any platform with a picatinny rail on top.
Here a picture of one on a Ruger pistol like the OP was asking about:
http://4.bp.########.com/-JRsHyCS4tR0/UAhEuae50yI/AAAAAAAAAE4/c7LhIDPtT6E/s400/IMG_2228.JPG
Here a picture of one on a Ruger pistol like the OP was asking about:
http://4.bp.########.com/-JRsHyCS4tR0/UAhEuae50yI/AAAAAAAAAE4/c7LhIDPtT6E/s400/IMG_2228.JPG