I got two muzzleloader barrels gave to me...
 

Their 28 inches long but 24 inches down the bores their
7/8 th's of an inch wide with a 3/8 th's bore opening,
Their hexagon but 5 inches is just round with a slot cut in to it. there both smooth bores..

My questions are what caliber are they, what all could I shoot out of them, and how would you go about fixing them up ?

Hope this helps to show how the slots are cut in them.

RE: I got two muzzleloader barrels gave to me...
 

you could build a custum reciever for the and maybe have them rifled and sized to something of a normal bore size if they are not already, i really have no clue!!!!