This seems.. simple?
replies(3):
Late 19th and early 20th century attempts at self-loading firearms were often ridiculous in their concepts; huge component counts, lots of tiny mechanisms, strange attempts at extracting recoil and gas energy, everything under the sun. The mechanisms engineers were crafting in literal garage workshops are stunning in their variety and staggering in their watch-like complexity. Some were genuine works of art.
Then the M1 Garand, the SVT-40, and afterwards the AK (under the economic pressures of WW2) demonstrated how much room there was to simplify and give various components double duties. Now, most modern automatic weapons derive from those designs, and the improvements since have been in the materials engineering: Stronger, lighter, thinner, and generally reducing the amount of steel to the minimum necessary.