they don’t necessarily need to be. ceramic could be used as
a projectile very effectively, and a case could be made of
polymer. would have less working pressure to work with, but enough to be effective close in.
Metallic-cased or jacketed ammo isn't the big problem, it's the barrel and breechblock parts. Ceramics have lousy tensile strength and would more likely turn into a fragmentation grenade than deliver an aimed shot.
I have a ceramic hand grenade sitting on the bookshelf. It is empty, of course.
It was made in the waning days of WWII in Okinawa. Steel was in very short supply, so the inventive Japanese made ceramic (unglazed porcelain, I think) hand grenade bodies.
The Hungarians used lengths of water pipe to disable Russian tanks during the revolution of 1956. They’d run out of a side alley and shove a long piece of 2” or so water pipe between the lower tread and the rear sprocket. Popped the tread right off, leaving the tank to run in circles.
“Where there is a will, there is a way” to make a weapon. When Milton Obote’s second UNLA government was overthrown in Uganda there was a story of citizens beating UNLA soldiers to death using old shock absorbers.
Maybe we should outlaw NAPA or AutoZone? /sarc