As I understand it the PRE 2020 .25 had mag holes with a diameter of .254 (6.46*) and that Stephen of AVS fame makes .254 slugs in various weights** in flat,dish,cup,hollow base, assuming that the mag (or barrel diameter) has not changed these should fit.
**weights, I am as yet not sure at what point Stephens .254 slugs become longer than 11mm.
I am SUPER interested in the 2020 barrel, the PRE did not shoot slugs* that well, I was told that Huben where putting a lot of effort into sorting this on the 2020 version.
*"6.46" size and other K1 info courtesy of Gregor kamensek.