My deer rifles are .54s, I shoot 80 gr of 2F and a 226gr patched round ball, at 100 yards my bullet energy is the same as if you put a 45 auto against the side of a deer and pulled the trigger, lots of stopping power, my load is good to about 200yards. You can load up to 120 gr of powder and reach way out there, I chose the most accurate combination for my gun.
There are two different kinds of barrels you can put on these guns, one with slow (1 turn in 66") twist rifling made for shooting patched round balls or one with a fast twist rifling (1 in 28" or 32") made to shoot a large conical bullet. I don't like the big conicals because of the severe recoil, these things can be brutal.
As far as completing one of these guns from a parts kit it depends on your patience, reference material and tools available to you and knowing how to use them. The problem with precarved stocks is the precarver makes mistakes, some really bad ones that you have to be able to correct.
I have made two guns from precarves and two from a stock blank, it was actually easier to make a gun from a blank than to make one from a precarve because the precarves were off center and poorly inletted and it took me more time to fix the flaws than it would have taken to shape the stock myself.
Here is a common flaw, a precarve lock inlet that is not in the right place, the pan should center my X mark on the barrel, in this case it is below the side flat of the barrel and just won't work. I had to glue in a shim in the inlet and drop the barrel deeper in the barrel channel to get the lock up where it is supposed to be, this took me at least a week.
Other flaws on this precarve were a ramrod hole that was improperly drilled that went to low in the forestock and had a bow in the channel and gouged out place in the forestock that I had to patch. This is on a kit that had $1100 worth of parts in it, unacceptable.