Fx wildcat mk1

Stick to pellets. The barrel in the MK1's is the original smooth twist - great for pellets but will not impart proper spin to a slug. The MK1 is also a "set and forget" set up. They can be tuned but the lack of external adjustments makes it a very tedious chore (shooting slugs requires a lot of tuning and testing). I have a MK1 in .25 and it is the most consistantly accurate air rifle I have. If it is shooting well for you now, I would just enjoy it as it is.