There are three companies which sell insurance for collectibles - including firearms.
http://www.historicfirearms.com/wordpress/?page_id=2
http://www.corevensguninsurance.com/
http://www.collectinsure.com/
Collectors Insurance advertises the most, but that does not necessarily mean that it is the best.
All of the rates are competitive but there are differences in the details - especially for large collections.