The Mondragon rifle, designed by Mexican Manuel Mondragon, was the first semiauto rifle adopted by a national military. It was not particularly successful beyond passing that bar, but it did show the way of things to come. Interestingly, it used a gas piston and rotating bolt – not a common mechanism at the time, but very common today.
See the full gallery of photos in the Vault – Mondragon Rifle.
I have no idea than the Mondragón was the first arm to be featured on the site(new site?) That’s great!