I have been building smokless muzzle loaders for many years, i simply use a Rem Rolling block action, a high pressure bbl in 45 0r 50 cal. 50 is the most popular.
They are fired with a 209 primer, 27 grns of 2400 powder. Tack drivers !!! Elk at 200 yds the barnes X muzzel loader sabo round will go through with lots of damage.
There is a pic of one i built on my website. web page
Birdseye maple stock,,Douglas Stainless bbl, half octagon, half round.