CCI subs always chamber tight in my Silhouette but they do shoot ok. You will probably find Eley the best but also probably the most expensive, I have shot loads of different stuff through mine.
All I would say is try a few, I use mine as a subsonic tool almost exclusively now and use Magtech 40g HP. Very similar to Winchester, a massive HP and very effective, plenty good enough for bunny bashing.
Tried loads of fast stuff in the past and Remington Yellow Jacket HP worked pretty well, I think they were around 32g can't remember, 1500ft sec. Never did find a fast load as accurate a