While not 100% plastic, these were offered just a few years ago to save weight for soldiers to be able to carry more ammo.
These are .223, they were also made in .308.
The company,(Natek), went out of business when the military didn't buy into it. These were not supposed to be reloadable, I did, but the bullet/neck tension was almost non-existent. The case was formed/molded around the bullet, then the powder filled case was snapped onto the brass base.
Active made an all plastic shotshell for many years. There was a steel ring imbedded in the base to strengthen it. Herters sold an all plastic shotshell with their "model perfect" spin off front end wad. What a joke! Special tools needed to form a crimp to hold the spin off over shot disk.