I have had great success with the Nielsen 17.5g slugs in a handful of different pcps. They work great in 2 of my FX 500mm STX Superior liners (standard). In a short barrel I dont think it makes sense to try and push the heavier slugs and waste a ton of air. If you can get the 17.5g going 850-1050fps in your Panthera, I think you will really like them. I know a lot of others have had lots of troubles trying to get decent groups with slugs, but I swap between pellets/slugs often. I pretty much only shoot Nielsens or JSB Hades since I am hunting almost exclusively. In my 22Cal setups I try to get the Hades going ~990-1050fps, so when I swap to the heavier 17.5g Nielsens I only drop down to ~950fps with no tune changes.