Plan diagram of 2-barrel <a href="https://en.wikipedia.org/wiki/1-inch_Nordenfelt_gun" class="extiw" title="en:1-inch Nordenfelt gun">1-inch Nordenfelt gun Mark I</a>.
Admiralty, UK
<a rel="nofollow" class="external text" href="http://handle.slv.vic.gov.au/10381/303770">Handbook for Nordenfelt gun, l-in. 2-Barrel, Mark I. 1886</a>