[Charles Martel]

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.

[marktwain]

Ceramics have lousy tensile strength and would more likely turn into a fragmentation grenade than deliver an aimed shot.

Materials Science is proceeding at an incredible pace. I think that composite materials with a majority ceramic component could be made strong enough to withstand low pressure loads like some shotgun, pistol, and rifle loads. Low pressure loads can still be effective.