Congratulations. The Huntsman barrels are very accurate barrels, but unfortunately the ignition (primer carriers) leaves a lot to be desired. You will not be able to shoot Blackhorn 209 powder with the OEM breech plug. There are other options for ignition. PR Bullets makes a .25 acp plug that uses .25 acp brass and small rifle/pistol primers. I have several custom breech plugs for shooting bare 209 primers that work well with BH209.
So, if you intend to use the OEM breech plug and primer carriers, I would recommend Triple Se7en loose powder, NOT pellets. Triple Se7en Pellets are not 100% reliable in these rifles with the OEM breech plug. If you want to shoot pellets, then Pyrodex Pellets are the way to go. They have a dab of real Black Powder bonded to one end to aid in ignition.
For a bullet, as Kurt mentioned, the 460 gr No Excuses sitting on top of 70-80 grains VOLUME of Triple Se7en FFG or FFFG, with a fiber gasket wad between them.
Another great bullet would be the Bullshop 460 gr NEx, they are a clone of the No Excuses. They can also be sized from .500" to .5045", and have a better lube. They are less expensive as well. The only problem is that Dan does not cast only part of the year, from October to April. The rest of the year he is working his other job.
If you need more help, just ask. There is also a lot of good information, if you don't mind going back and digging it up.