I've shot 25gr H&N and the 26gr FX Hybrids with good success. The barrel isn't choked so that helps a lot.
I stuck with 25-6gr slugs because it lets me use the same tune I shoot my 25gr pellets with.
I really need to slug my barrel so I can actually buy the correct size slug. Once I know that I might try some heavier slugs for longer range shooting. Right now I can pretty reliably hit a spinner @ 87yds(farthest I can shoot at the moment) with a 36yd zero. To really try I need to re-tune and re-zero which I won't do until hunting season is over.
Have fun!