Never in rifle or pistol ammo, but I have had shotgun shells made entirely out of plastic before (no brass cap on the end). They worked fine in pump guns, but didnt do too well in autos. Federal made them back in the early 90's if I recall right.
ACTIVE made them. Ballistic sold them.I have a 100 new I wanted to load with slugs.active went out of business right after I got them.1993/4.I think they were made in Virginia.they were owned by couple companies.
I had some 5.56 fmj grey plastic case ammo. The bottom of the case was brass about 1/4" up. They were alot lighter and functioned fine. You just could not fire standard brass and then load the plastic ammo until after the rifle had cooled down or the plastic case would get stuck and the brass bottom would come off. I don't remember the brand but I do remember they had a website.