USMC experimented with brass 12 ga ctg in Viet Nam because the paper ctg was prone to swelling. The full brass still required to be wiped down w/oily rag to avoid corrosion. They only lasted a short while before they were replaced with the plastic ctg which was becoming commercially available around that time. Military has used plastic ever since. I have some 12 ga 00 Buck from the early 70s that is still useable. It's Federal plastic.
Oh, and the Russians also had a full-brass 12 ga round, but I know nothing about it. Also, S&B (Czech) made them for the military years ago, as well.
Now I have a curiosity of my own. I found a full brass .410 case in the desert, a few months ago. It has no brand on the headstamp, only "410" and nothing else. I had never heard of a full-brass .410 round before.