Doesn't make a lot of sense to use the Ruger target grips on the snub version. They do feel very good in the hand, and I'd use them on a four or six-inch gun.
Pachmayr makes/made a round-butt grip for the Speed-Six that feels great and hides better. Don't think they have a square-butt version that size, though.
The rub with the design is that many I've handled had a slightly loose crane fit, and you can feel some motion. I think that's why Manurhin added a sort of crane lock to their version of this gun, and why Ruger designed a front cyinder crane lock as he did on the GP-100.
I had a Security-Six with six-inch bbl. that I loved. Had to sell it and other items when my ex let me have my daughter and we had to move to a better school district.
Like the M-66 with that barrel, it gives full .357 power with just moderate bulk. It fit a Safariland M-29 holster that'd also take the equivalent M-19/66.
Assuming that a particuar gun is properly fitted, I think these Rugers are terrific. I suspect the loose crane issue has to do with the manufacturing process and the lack of hand-fitting cranes to indvidual frames, where tolerances match. When they do, all is happiness.
Be aware that serial numbers beginning with 150 use different grips than later guns. You can tell them also by the sloped look at the top of the grip, whereas later guns look like the S&W handle there. Controls recoil better.
I much admire the GP-100 and SP-101 and have both. But I'd buy a Speed-Six or Sec-Six with a four or six inch bbl. in a heartbeat if I had the money when I found a nice one in stainless.
I have heard that Ruger no longer services these guns, as they're out of parts. You might want to think about that.
