Personally, I never cared much for the 600 Jr. I never liked the way it sized hulls. I actually prefer to use one of the collet resizng presses. Either the Sizemaster for a single stage, or the 8567 Grabber for a progressive.
Asking what kind of powder to use in a shotgun is kind of like asking what color to paint your house. My question to you is, what kind of load do you want? What do you want to shoot with it? Is it for small game? Pheasant, Dove, Duck, turkey, deer, home defense, practice, what?
My best advice is to purchase one or two of the different shotshell manuals out there. The Lyman manual is a good start, it has lots of useful information for a shotshell reloader in it. There is usually some info in there about shot ballistics. As for powder, I'll give a rundown on all the powders I've used and for what purppose.
Bullseye - Light 7/8 ounce 12 gauge target loads to 1200 fps
Red Dot - Light 7/8 ounce up to 1 1/8 ounce target loads to approx 1255 fps. If you use Red dot for the lighter shot weight, velocity can go as high as about 1350 fps (7/8 oz).
Green Dot - Light to medium duty powder, 1 - 1 1/8 ounces, up to 1300 fps.
Select - Light to medium duty powder, 1 - 1 1/8 ounces, up to 1300 fps.
Unique - Medium burn rate powder, 1 1/8 to 1 1/4 ounces, up to 1300 fps
Herco - Medium to heay shotshell loads, 1 1/8 to 1 3/8 ounce loads. Up to 1300 fps, give or take, lighter shot loads can be driven to 1350 fps.
Blue Dot - High velocity hunting powder - 1 1/4 - 1 1/2 ounces in 2 3/4 shells, 1 3/8 to 1 7/8 ounces in 3" shells. Velocities from 1200 to 1400 fps.
700X - Similar to Red Dot/Select
800X - Similar to Unique/Herco
PB - Similar to Unique
SR 7625 - Similar to Herco
SR 4756 - Similar to Blue Dot
Titewad - Similar to Bullseye/Red Dot
Titegroup - Similar to Red Dot
HS6 or W540 (W 540 is discontinued) - Similar to Herco/Blue Dot
Clays - Similar to Red Dot
International Clays - Similar to Green Dot/Select
Universal Clays - Similar to Unique
Longshot - Similar to Blue Dot/SR 4756
Ramshot Competition - Similar to Red Dot
There will be some discussion probably about the powders, these are my experiences with them. In my nearly 20 years of reloading, I've tried all of these powders and still have a fairly good stock of a lot of these that I use for various things (including handgun ammo).
I held the list mostly to one gauge, but the slower burning powders are also good for the smaller gauges (20, 28). The fastest powders are at the top of their respective brands. Red Dot, Bullseye, Titewad, Titegroup, 700X, Ramshot Competition are all on the faster burn rate scale and while they may be useful for very light 20 gauge loads (usually 3/4 ounce target loads), you tend to use a more medium to slower rate powder for 20 and 28 gauge. the 20 uses more like PB, Unique, 800X, Herco, Blue Dot, HS6, International Clays, Universal clays, and Longshot. The 28 uses the slower end powders like Blue Dot, Herco, Longshot, SR 7625, and SR 4756.
About Primers, there is a relative "hotness" to shotshell primers also. The list is roughly:
Mildest: Remington 209, Winchester 209
Medium : CCI 209, CCI 109, Fiocchi 615
Hottest : Federal 209, CCI 209M, Fiocchi 616
When I load shotshells, I generally keep the components almost as listed. If I don't have a particular wad in stock, I might make up 1 box of shells with a different wad and not use the maximum powder charge to see what kind of results I get. The problem is, the data was meant for a specific wad, hull, etc.
If you change one component, it may take a little more or less powder to make the claimed velocity in the book. I've substituted similar looking hulls and gotten good and bad results. If I'm curious if something will work, I'll only change one component, load a box up and go shoot them, I might even chrono the load to see if it does what I want it to do. If I get squibs, I'll figure the data isno good and then move on. If the load looks promising, I'll write down the data for later when I might want to duplicate it again.
Sometimes I've found substituting say a windjammer wad for a AA wad might make the pressure drop and I need a little more powder to make the same velocity. So I increase the charge by a grain and try it again until I'm satisfied with the load. Yes you can blow yourself up, but if you are as careful loading shotshells as you are loading metallic, then I don't see why you can't substitute components. The guides pretty much tell you not to do it to cover their behinds. There is some wisdom in there and I wouldn't change a load so much that the only thing that is the same is the hull, or just the powder. If you take things one step at a time, you will have some failures and you might find a better load. That's the part of reloading that I like.
Another avenue you can explore if you are truely interested in shotshell reloading is to take a look at the web site called Ballistic Products". They have even more data for not very commonly available stuff. Hunting wads that come pre-slit or ones that you can slit yourself. They also have a wealth of data for steel, Bismuth, heavishot, etc. I too load steel shot and while I don't mind loading steel, that Kent stuff has gotten so good and so fast, it almost doesn't pay to reload steel anymore.
Vince