Ruger is drilling mounting holes under the rear sight? That's news to me. The Ruger sight is so tiny, compared to the long S&W sight that runs almost the entire length of the top strap. Hole spacing would have to be like 1/4th of an inch. Most of the base would be unsupported and cantilevered over the frame. Very curious.
Brownells sells a
Weigand base for the SBH/SRH that should fit. $30 seems high for a little piece of aluminum. It required you to remove the rear sight and drill and tap the top strap. Those holes would look like sin if you ever wanted to go back to just the rear sight.
No Drill mounts are temping but I've never seen one that would really hold up to a full bore revolver cartridge or provide the stability the Ruger Bisley warrants for longer range shooting.
That said, I think there is a company that makes a no-drill Blackhawk base that attaches to the rear sight
and the front sight - so it runs the entire length of the barrel. Which is perfect for dot sights, since they're so short and work better mounting farther forward on the gun. Can't remember who makes it, maybe B-Square or Power Custom. Full length mounts like that are pretty expensive, like $100.
Personally, if it were me, I'd look for a base that I could drill, tap, and attach to the barrel (with little epoxy added for insurance). Something with a round bottom that was close to the barrel curvature, took weaver rings, and had a U-shaped (or low profile) center channel. That way you could leave the base on a still use the original sights. Such a mount won't be advertised specifically for the Blackhawk, it would be a generic mount or a mount offered for some other gun. It would take some looking but that type of base is common to every mfgr.
I had two S&W revolvers set up that way years ago, a 686 & 617 both with 6" barrels. Loved those guns. Unfortunately I lost them in a very convoluted trade, the details of which I won't go into. The base I used IIRC was made by Tasco, and was marketed for some other gun but fit the Smiths like a glove. The bases were aluminum with a silver finish and cost like $7 ea. Weaver probably makes a two dozen bases that fit the bill, the only difference is the spacing of the screw holes and since you'd drill your own it wouldn't matter. -- Kernel