Since we’re on the topic of triggers, here are a few links related to trigger design in addition to some general interest links.
The first is a PDF of the Remington-Walker trigger.
and an explanation of the Savage Accu-Trigger.
An interesting calculator to help figure gas pressures that may help in designing gas operated firearms.
Of course you’ve probably heard of making an AR-15 on a 3D printer by now.
Finally, while the explanations are basic, there are some excellent illustrations of firearms actions in the HowStuffWorks Machine Gun article.