As of 2005, there are really two entire categories of "Blackhawk".
Since 1973 all "Blackhawks" and "SuperBlackHawks" have been built on the same 44Mag-class frame.
Except for one: the 50th Anniversary 357Mag Blackhawk ("Flattop").
This piece is really special. It reverted to the pre-'73 "mid frame" size, very similar in proportion and feel to the Colt SAA. If you have one re-chambered in 45LC, it would NOT be able to take the "45LC+P Ruger ONLY!!!" monster loads by Bufallo Bore and others.
So why consider it?
Because when Ruger designed this new mid-size frame, they did a really excellent job. For one thing, with the loading gate open, it will "click" each time a cylinder bore is lined up to recieve a round. The large-frame guns don't do that, they "click" when the cylinder is halfway between in the gate.
Better yet: the mid-frame cylinders are bored one cylinder bore at a time using a single drill bit sequentially on each bore. The large-frame guns are bored six-at-a-time and there is sometimes enough variance in cylinder bore and throat dimensions between bores to affect accuracy. The process used to make the mid-frame cylinders eliminates this possibility, and since it's easier to check a single drill bit once in a while, we're seeing much better cylinder quality.
The barrel/cylinder gaps are running medium-to-tight (.002" to .004" usually) on the mid-frames and all other fit and finish issues seem to be good to excellent. I think these are the best guns Ruger has made in a long time.
The 50th 357 is the only adjustable-sight mid-frame Ruger. The Ruger "New Vaqueros" are fixed-sight mid-frames...so my New Vaquero in 357 is the same gun as the 50th 357 Blackhawk but with fixed sights...which on my gun have been improved after the fact with a dovetailed windage-adjustable front on a custom base and a hogged-out rear channel to match. I did the sight mods out of personal preference, NOT because they were off from the factory - they weren't.
Re-chambering: the new mid-frame guns are believed to be strong enough to take the 41Mag or 10mm. I know of re-chamberings to 41Mag so far with no problems. The 44Spl is also a popular big-bore mod with these. In 45LC they work fine as long as you stick with loads that can be fed into a Colt SAA of modern manufacture - figure a 255gr hardcast at about 1,000fps, or a 200gr jacketed at 1,100. 270 hardcast at 1,000 is pushing it but doable with careful handloading. The Ruger mid-frame is likely just a hair stronger than a 2nd/3rd gen Colt SAA or the various Italian clones.
Parts interchange: a lot of pieces from the Ruger larger-frame series bolt onto the mid-frames. My New Vaq sports a SuperBlackHawk hammer. I could also switch grip frames to anything that will fit the larger-framed guns, with a bit of tweaking: you have to also swap mainspring, mainspring strut and "keeper". I'm considering lightening my gun with an aluminum grip frame and ejector rod housing from a standard post-73 Blackhawk. Cylinders on the mid-frame are shorter and hence base pins are different too. Belt Mountain has replacement oversize pins for the mid-frame guns.
If you can live within these limits, which includes all factory 357Mag ammo out there in that caliber, the new mid-frame Rugers *rock*.
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Whatever Ruger you get, you'll get a transfer-bar safety. Highly recommended for both safety and six-up carry. You should also consider either swapping the base pin cross-latch spring so the base pin can't possibly jump loose under recoil, or use the set-screw feature on a Belt Mountain pin ($25 or so) to solve this issue that way. The cross-latch spring is just a few bucks, or comes free with any spring kit (less than $20). Letting the base pin jump is about the only way to hurt a Ruger and in my opinion it's so cheap to eliminate that it's worth doing. Others say "don't fix if it ain't broke"...
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There is also a 50th anniversary 44Mag "flattop" Blackhawk. It's a good gun but is built on the larger frame...