I presume that you ran Jackrabbit as a server and then accessed it via something like http://localhost:8080/repository/default/. Is that the case?
I don't believe that ModeShape's WebDAV service supports HTTP range requests. If you're really interested in the feature, you could log an enhancement request and perhaps even help implement the feature. If you configured ModeShape to store the binaries on the file system (rather than in MongoDB, Infinispan, or in a relational database), then the Binary value's resulting InputStream should really be just a FileInputStream implementation (wrapped by a BufferedInputStream) that directly accesses the file. That means the server should be able to efficiently call "skip" on the InputStream. At least in theory.
Of course, you could always write your own HTTP servlet that used the JCR API to build the responses. That may be more than what you're interested in doing, though.