Take your time and keep a log journal. Expect to take about 4-5 minutes in-between each shot to allow for the barrel to cool depending on ambient temperature.
Of the powders you have, Pyrodex will be the most fouling, APP the least. I am not a fan of Pyrodex but it does work pretty well.
Follow the CVA loading data for your rifle. CVA says FFG only, but I have shot 3FG in up to 58 caliber rifles since 1970.
http://www.cva.com/warranty/CVA%20In-Line.pdf
According to the manual, your maximum load is 100 grains FFG Black Powder, APPor Pyrodex. Or if you wanted to shoot FFFG (3FG) then 90 grains would be your max in black powder, pyrodex or APP. Rule of thumb is you reduce FFFG charges by 10% for FFG loading data.
Triple Seven (777) requires an additional15% load reduction due to increase in pressure and it's more powerful. So if you shoot 777 FFG, then your max load is 85 grains for your rifle.
Imight suggest you not use777 3FG (FFFG) as 777 requires a 15% load reduction because it's more powerful and with any 3FG powder, you should also reduce loading data by 10%. So if you wanted to shoot the 777 3FG and stay within your safe load, your max charge would be 75 grains. That does not leave you much in the way of variability in order to determine the most accurate load with a given projectile.
You can shoot 777 2FG (FFG) in which case your max load for your rifle will be 85 grains 777 2FG (15% load reduction).
[ul][*]Start out at 50 yards and focus on grouping there first about 2" high. Forget about sight adjustments at first, just try to get the best grouping.[*]Pick one projectile, cap/primerand one powder to start with.[*]Take your time and focus on your loading procedure. Also use a powder measure in 5 grain increment as you will be able to much better tailor the most accurate charge.[*]Start with a moderate charge of FFG powder70-75 grainsand see where the load prints. (If you elect to shoot 777 3FG, then start at say 60 grains and your max is 75 grains). Shoot 3 shots and see if they are grouping at all, if not then up the charge in 5 grain increments and 3-shot groupsup to your max loading data.[*]Expect to change targets alot. Your best bet is if you can set up 4 or more targets at one time.[*]Cleaning in-between shots. You need to choose if you are going to POI from a clean or fouled bore with only occassional cleaning. I only group from a clean bore and that is the POI I hunt with. Whatever you choose, make your procedure as consistant as possible. If cleaning in-between each shot, then use a 50/50 mix of alcohol and windex on a dampened patch. One or two before each shot followed by a dry patch should do the trick.[*]Record which charge levelshoots the best group with a given projectile at 50 yards them move out to 100 yards and see if the groups are holding. Adjust 5gr up or down at 100 yards if needed. Adjust the sights for your desired 100 yard POI.[*]Then go back to 50 yards with the same powder, switch projectiles and start the process over. You may find that one projectile will shoot similar to another so you don't need to shoot as many times to figure your groups.[*]FYI - If you don't have a spotting scope, you will have a hard time seeing your POI at 100 yards.[*]After determining which loads shoot best with each projectile. Switch powders and start the entire process over. Every time to change something (Powder, projectile, primer) then you need to re-verify your loads.[/ul]
Of the projectiles you mentioned I would prefer to shoot the SST or the Great Plains Conical because of their exceptional performance on game.
Also, you will find the the 250gr SST will load much easier than the 300, so FYI. The 300 grain is longer in length and greater force is required to load.
Another nice loading and shootingsaboted round is the Hornady/Thompson-Center45 cal XTP Mag.
Good Luck and let us know how things go!
Tahquamenon