The DC museum job market is extremely competitive.  Many hires are done
via National (and sometimes International) searches.  Many available jobs
are open to outside applicants, though applicants from inside or other
museums in the area certainly have an advantage sometimes.  Museum
experience of some kind, whether volunteer, internship, or employment,
is a plus when applying for any job, DC or otherwise.  Also, all museums,
DC and otherwise, have budget constraints and there are often not that
many openings available.
I suppose when I got my permanent entry level position it qualified as
an internal hire.  However, I had a summer job, a contract job, and
2 different temporary jobs (all at fairly low pay) before I got the
permanent job, which was essentially at the same pay level as the last
temporary job.  (Not to mention the jobless period when I was on
unemployment after one of the temporary jobs ended ....)  This is not
an unfamiliar scenario.  I stuck it out because I really wanted a
museum job but it was not easy.  I am afraid this is the reality for
most museums in big cities.  That said, big museums have job openings
on a regular basis, especially entry level jobs that are not in the
curatorial "track."  These positions are often filled by those on the
outside of the institution, who are able to establish a track record
and apply for better jobs that come up, either in DC or other museums
in other parts of the country.