Doug K,
Charles Lancaster was 'long-gone' when your rifle was made.
He died in 1878 and was replaced by his brother, Alfred, who died in 1890. In 1892 the company was purchased by H.A.A. Thorn who then operated as "Charles Lancaster"until his death in 1914.
"Slugging" the barrel involves driving a soft oversize lead ball
through the barrel. The groove diameter is thus established. While your barrel may not have traditional 'grooves,' you'll still need to know the maximum diameter for proper bullet selection. Your proof marks will likely include a "39" which is the typical bore size for .500s.
Paper patch bullets aren't necessary.
As to reloading dies, 7/8"x14 are available for the .500 but you'll be better served by 1" x 14. Proper sizing of the head of .500 basic cases can be a problem with 7/8" diameter dies.
