Basically you have two options for powder to use in your new rifle. Either blackpowder, or a blackpowder substitute. Real black powder is available in several different grades 1fg (very course) 2fg (finer) 3fg (really fine) 4fg (super fine). Generally the courser grade like 1fg is used for shotguns, cannons, and large bore rifles or muskets. 2fg is probably the best all around choice for rifles and is the grade I tend to use the most. I've used 2fg powder in rifles from 45-58 caliber and also in my muzzleloading shotguns in 10-12 guage. Some people use 3fg in rifles up to 45 caliber, but I tend to find that the finer grades of powder are best used for pistol and small bore rifles like 32-36 caliber, though I can't see where using sensible charges of the finer 3fg would cause any harm in your rifle. What the finer grades do is raise pressure levels which also generally speaking, increases velocity. 4fg powders are pretty much reserved for priming flintlocks or small bore pistols.
As far as black powder substitutes there are now several very good ones available. Pyrodex is the most common the grades I use the most are RS and P. RS is the 2fg version and P is the 3fg. Other substitutes includeHogdens 777 and also a few others such as Black Canyon and Clean Shot. Some of these powders are advertised as non-corrosive and non-fowling. I can't say as I have had much experience with these. I've used Hogdens 777 and it is clean burning but it is still corrosive, though it's pretty simple to clean up. I'd say whatever powder you elect to use it's always a good idea to clean the gun immediately after your shooting session is over otherwise you will likely have problems.
