Shotgun Shells
My first foray into printed ammunition was the humble 12ga shotgun shell.
Filament
The very first design had a large rim to facilitate use in the pipe shotgun. I started out printing these in PLA, but they'd shatter when used in 3/4" Schedule 40 steel pipe. This is probably insurmountable problem with PLA, since pipe is much larger than a purpose-cut 12ga chamber.
Later versions of 3D printed shotgun shells use Taulman Bridge Nylon. It's flexible enough to be fired in a proper 12ga chamber and pipe without shattering.
Resin
Resin shotgun shells are some of the best looking shells I've produced. Using Siraya Blu resin, I got several nice looking shotgun shell hulls.