It would depend on what you have available. The M1842 Springfield was originally a smoothbore and thousands were rifled in the 1850's. A round ball load like the one Musketman describes would probably do a great job.
When the original '42's were rifled, oversized Minie bullets ( called expanding balls by the Ordnance Dept.) were used in them. They averaged, depending on the depth of the cavity, about 680 to 710 grains in weight. The powder charge was set by the Ordnance Dept. for this bullet at 70 grains of musket powder which is about the same as ffg., maybe a tad coarser. You can experiment with lighter or heavier loads though you won't want to go much heavier due to the need to keep the recoil at an acceptable level. If your musket has the proper long range sight on it, you'll probably find that it is sighted for use with a Minie. 69 cal. Minie molds are probably harder to come by, though with more repro .69 rifled muskets on the market now, there is the chance that there are more available than in recent years.
Musketman has a pretty good load there to start with and he knows what he is talking about. As with any round ball load, there is a lot of room to play with and you can work up whatever works best for your particular gun. As with any military musket, there is a chance that it will shoot high with the Minie. It may be perfect with a roundball. In either case, you may have to do a little filing of the sights. Good luck. It sounds like you have made a great purchase.
