A diagram from an army training manual of a <a href="https://en.wikipedia.org/wiki/belt-fed" class="extiw" title="en:belt-fed">en:belt-fed</a> <a href="https://en.wikipedia.org/wiki/machine_gun" class="extiw" title="en:machine gun">en:machine gun</a>.
US gov
<a rel="nofollow" class="external text" href="http://www.enlistment.us/field-manuals/fm-23-14-M249-light-machine-gun-in-the-automatic-rifle-role.shtml">Field Manual 23-14 - M249 Light Machine Gun in the Automatic Rifle Role</a> Chapter 3.