Unless your Barn Owls are somewhat larger than ours I wouldn't expect a Barn Owl to be able to take anything larger than what we term "pocket buns" ie a baby rabbit on it's first venture outside the burrow. Even then I think a Barn Owl would struggle.
Biggest bird of prey we have around here is the Buzzard, they will take adult rabbits, wing span 4 to 5'. The UK has a few places with Golden Eagles, they will take small lambs, wing span 6'+. Bald Eagles are bit larger maybe 7'+ wing span.
Something is, a Barn Owl is