Thanks for that insight .Just a bit of advice concerning the 357 Magnum in K frame S&W.
Open up the cylinder and look at the underside of the barrel extension inside the frame window. You'll see that it has a flat machined in it to clear the gas ring on the cylinder. You'll also see that the forcing cone of the barrel is thinned quite a bit where that flat is machined. This thin spot is the one weakness in the K frame S&W concerning the 357 Magnum and this weakness didn't become evident until the 70's when light bullet 357 Magnums came on to the market.
So, here is my advice concerning shooting 357 Magnums in your model 66. That is to ONLY use 158 grain lead bullets, personally I would recomend semi wadcutters. It is the predominate ammunition that was in use when the 357 Magnum was first introduced in the K frame. Shoot a modern 125 grain jacketed hollowpoint in 357 Magnum and you may find that you've split the forcing cone with just 50 rounds fired.
BTW, the above ONLY applies to the 357 Magnum, the 38 spl. of 38 +P just doesn't generate enough kinetic energy to split the forcing cone. So, if you want a good defense load just choose one of the modern 125 gn. hollowpoints in either 38 spl. or 38 +P, such as a Speer Gold Dot. I also expect that you'll find the 38's much more comfortable to shoot in volume than 357 Magnums, IMO the K frames are a bit light for comfort with the Magnums.
As for your model 66, it's a Stainless Combat Magnum. Which means that it's one of the finest balanced revolvers ever made and balance does matter in Combat. It's why it was a preferred Police sidearm before the bottom feeders took over.