Gun Control & RKBA

are banned by Federal law as armor-piercing, IIRC. Most of the practical alternatives to lead are similarly banned as AP, because they are harder than lead and therefore more penetrative. The virtues of lead are its density, softness, and abundance. Gold would also work, but obviously is cost prohibitive.

Solid copper is used in some big-game hunting loads (e.g. Barnes X-bullet) because it penetrates like crazy, since it doesn't deform much on impact.

Composite projectiles have very short range, as do aluminum projectiles. The old Thunderzap rounds (LE only) were aluminum, as I recall. I wonder if a .357 could drive a lightweight composite or aluminum projectile fast enough to penetrate NIJ IIIA armor; I know the Groupe d'Intervention de la Gendarmerie Nationale (that famously took down the Air France hijackers in the '90s, among other things) used to use .357's with AP bullets.