gnu -

There have been a number of "plastic bullets" going quite a way back, and your description sounds more like one of those. One such was called a "somethingorother-Jet" (might have been "gyrojet?") and had a minimal "case" that burned out completely, with the propellant mostly in the bullet so that it was more like a rocket than a normal slug. It had "fins" so it could look like a rocket and did have sort of a "triangular shape."

The blather claimed no cartridge to eject for rapid reloading, very light weight ammo so troops could carry a lot more, and "advanced technology." The advertising omitted the minor problems that nobody could hit a target with it very often, and sometimes the ammo exploded in the can instead of in the gun; but it made a splash with the news writers.

There were lots of others, so it's just a guess whether this might be the one you recall.

John