I was going to say the same thing as
stoti but I have a Mark1. All of the dealers I checked with, and that was many, were out of stock on the .25 600mm slug liner A which is the one you want if your gun has the 600mm barrel.
http://penchetta.com/fx-stx-slug-liner-a-25-cal-600mm/ May have one they don't say out of stock .
I wish you luck. Don't forget you need to tune the gun for the slugs to group well and it may take a few hundred rounds to get the harmonic tune just right so buy lots-o-slugs. The NSA and VarmintKnocker slugs group outstanding with the slug barrels. Dealers with slugs that werq well with the FX liners will usually say" for FX slug liners".
Advantages are greater FPE at medium to long range, greater expansion of hollow points, half the windage correction of pellets, and excellent performance at ridiculous ranges. Slugs will usually outperform pellets in every way from 100yards and beyond.