I would never advise a spaniel for air rifle shooting, as they are a hunting dog, and allhough they can be trained to walk to heal, it is not where they naturally should be, they are much better with shotgun in hand
For air rifle shooting a retriever breed would be so much better suited. But as you want the dog to catch rabbits too, then as has been said, a lurcher seems the obvious choise, and id recomend a collie based type as they are steady and easily trained to walk to heal etc etc...
Just my opinions based on my experience, as i keep gundogs as well as lurchers and other dogs.
good