Way back before the internet, the Army made training films. In 1966, the XM-16E1 was the new hotness and the Army wanted to train a million draftees how to use it. They produced an Operation and Function Training Video which is still one of the most informative videos ever made. It uses a cut away version of the M-16 rifle to show the cycle of operation.

https://youtu.be/hg7uITzHpao

Thanks to the guys at

VDMA Videos for bringing this to us.