Tried a lot of pellets....at one time had a bunch of "left over" 5mm's and one of the rifles was a Sheridan C
Consider that the 5mm is about the LEAST popular caliber...the fewest number of pellets still in production....doubt you'll see "slugs" offered.
Whatever your defination of slug, none were cast or swaged "bullets".
Sheridan ("peep"):
Were a good number of PCP 5mm's at the time...couple of them still with me.
I do not see any suppliers of .20 slugs in this .pdf or a google search:
While a few months or more out ..... Had heard personally from Nick at NSA he was going to do .20 cal at some point ( this being @ 1 1/2 years ago )
Then here recently ( Within the past month ) Nick stated to me that .20 was in the pipe now & getting dies made estimating within a few months some slugs in .20 could become reality.
I would love to try a .20 slug in my TalonP. Right now I am shooting SamYang 23.7 grainers and loving them.