So the 4in barrel seems to be most popular, any reason?
Also, everyone keeps saying single action, but with double action you get the option for both yet faster reloads, correct? with all the benefits of the double action why is single action brought up so much?
I believe KMAX hit the nail on the head: the 4" barrel is the most versatile. It has the ability to be [relatively easily] carried concealed, it is lighter to pack on the hip than a 6" barrel, yet it has a longer barrel for better shot recovery and velocity than a 3" barrel.
Basically, Longer Barrel = more velocity for round, = higher energy transfer to intended [or unintended] target, = faster shot to shot recovery due to extra weight reducing barrel rise, = longer sighting plane to make it easier to aim more effectively.
Shorter barrel = less velocity for round, = lower energy transfer to target, = more muzzle rise due to less weight, = shorter sighting plane to make precise aiming more challenging.
It is often said that a longer barrel is more accurate. This is not necessarily so. However, the longer the distance between the sights the better for your eye to make out nuances of aiming.
The 4" is the best blend of both. It is fun at the range for all day shooting, like a 6". It moves less in the hand than a 3". It allows more powder burn, for better ballistics.
As I had said, I'd want a 4" as I would want to be able to pack it, or nightstand it, as well as pull it out of it's crate at the range.
If all you intend is to use it at the range, then the longer the barrel the better for terminal ballistics, for muzzle control and for sighting plane.
As to DA/SA vs SA- you are right, a DA/SA revolver does give you the ability to fire in SA mode all the time, if you so choose AND it allows faster shell dump/reloads than a SA revolver.
I have both.
I will say it is much easier to fine-tune a SA revolver's trigger/hammer to gain an awesomely crisp trigger than it is in a DA/SA revolver.
I've gotten all of mine where I want them, but it took me a LOT more work on the GP100 than the Blackhawk.
The good thing about the extra time between firing [length of time to reload] is that it gives you more time to think about what you were doing the last cylinder worth: shot placement compared to sight alignment, 'pulling/pushing' trigger, flinching, etc.
As you reload you can reflect on the good/bad of the last loads and then focus on fixing them.
The ability to use a speedloader and rapidly replace the ammo in a DA swing-out cylinder is slower than an autoloader, but it is fast enough you may focus more on the mechanics of reloading and getting back into your firing stance than you will on your shooting technique.
You stated you wanted a larger revolver [hence anything less than 4" is out, and I tried to steer you to 6" or greater] and it was only for range time.
With those as the only parameters, the advice to get a SA revolver seems like a good fit. There was no indication of it serving night-stand duty, carry duty or anything else- as such, speed of reload was not an issue in the advising process.
Also, if self-defense isn't an issue, than neither is a DA trigger pull. Some intend to use their GP100/S&W 686 for self-defense but they intend to cock the hammer manually each time. If THAT is the intent, then there is no reason for all of the DA lockwork in the gun- a SA revolver does that perfectly, and was used for a century for defense against critters in the west [both 2 and 4 legged].
Lastly, as a range-only fun-gun, many pointed out that you can get a convertible SA and it gives you 3 calibers to shoot: 38Special, .357Mag and 9mm. You can't do that with a DA/SA revolver [well, not with most, and not for less than $1000].
SO, for a range only gun, the SA Blackhawk 6.5" barrel seems to fit all the parameters you listed, as well as providing you with caliber options, for a lot of days in the sun.
I personally would hold onto my GP100 4.2" [true length, but everyone calls it a 4"] over my Blackhawk, but I may revisit that if I were not allowed to own a semi-auto ever again.
If we lived in a revolver only world, I'd probably keep the Blackhawk for cheap ammo [you do know most 9mm is about 60% the cost of .38Special and 50%0 to 40% the cost of .357Mag ammo, right? Or that you can get 2 boxes of 9mm for the cost of 1 of .357Mag?]