The Blackhawk is loosely patterned after a Colt SAA of 1873 - "loosely" because it's larger, capable of coping with WAY more power, has adjustable sights, coil springs and a good safety that prevents discharge if dropped (if made after 1973 - older ones can be retrofitted with the safety for free by Ruger and most have been).
SAA-pattern guns have some advantages even today over DA revolvers:
* SA frames are smaller than their equivelent-power DA cousins because the cylinder doesn't swing out - there's a whole mechanism "missing" on the SA that gives them a slimmer profile.
* The "fixed in place" cylinder is also a tougher design and can result in greater accuracy, at least when two guns cost about the same amount. Each time you swing the cylinder closed on a DA you're making a new "alignment" between barrel and cylinder and it can vary by a hair or vary a lot over time or if that DA hinge (called the "crane") is bent.
* Therefore the SA is tougher and lighter than an equivelent-horsepower (bullet energy/caliber) DA.
* Because the SA grip was designed for one-handed shooting on horseback, for MOST people it copes with big power better.
* If you study the Ruger catalog online, you'll see that Ruger will sell you an SA revolver in 44Mag that is significantly lighter than their lightest DA.
* Being tougher, a lot of people like SAs as a woods carry/outdoors/critter defense type of gun.
* IF you're willing to put in the time to really learn the SA, it actually has the fastest first-strike speed (fastest first shot out of the holster) of any handgun type made.
* Finally, in areas where you're likely to encounter "gun unfriendly" people on the trail/forest/etc. an SA will have a less "aggressive" appearance (to most non-gunnies) than a DA or autoloader. It's not a major difference but it is there.
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One more thing about "Blackhawks": all of the ones made after 1973 (also known as a "two screw" for the doubled cross-pins through the frame between hammer and trigger) are built on the "large frame" capable of handling 44Mag horsepower or above. This includes those built with lesser calibers such as the 357.
There is ONE exception: the 50th Anniversary 357 Flattop made in 2005 and still seen for sale now and again. This gun was built on the same smaller frame as the New Vaquero and cannot be set up as a 44Mag. Build quality on these "mid frames" was exceptionally high though, so if you can do what you need done in 357 horsepower levels, these are really sweet. In terms of size, the mid-frames are very close to Colt SAA 1873 dimensions. My Ruger SA is a New Vaquero in 357 with a sights upgrade and lowered hammer bolted on from a SuperBlackHawk, set up as a "streetfighter CCW SA".
The "Old Vaquero" (really just called "Vaquero" by Ruger) was a fixed-sight gun built on the larger post-'73 "44Magnum class" frame.