Hi Rik mate, welcome to the forum,
Just to extend onto what Sweeney has said; for a basic Break Barrel springer for rabbits your best bet is probably a general domed pellet.
'Superdomes' are very popular as they hit quite hard and as a rule are very accurate. Personally I use Air Arms Diablo fields which are a bit of an all rounder/hybrid pellet and RWS Superdomes.
The Air Arms are very accurate and fit the barrel really well, they come in both 5.51mm & 5.52mm, the RWS Superdomes are also nice and accurate and pack enough punch to kill most small quarry in one hit.
Hope thi