Why do shotgun shells have brass or steel encasing the base while

If shotgun shells are made of plastic, is there a risk that they might melt inside the magazine tube after prolonged shooting? Or is this not an issue with most modern shotguns?

Cartridge Advancements

What are TMJ and BEB Bullets?

Why are shotgun shells all rimmed? - Quora

Base Wad Separation

Why are shotgun shells made of plastic? - Quora

How Do Shotgun Shells Work? What Are Shotgun Shells Filled With?

Soft Point Bullets - What Are They & Why Use Them?

Do all 12 gauge shotguns take the same shells? - Quora

The Shotshell and its Development

Why are there no low brass, 3.5” 10 gauge shells loaded with #8 birdshot? - Quora

Why are shotgun shells all rimmed? - Quora

Ammunition, Bullets, Shells & Cartridges

Ammunition Selection Stock & Barrel Gun Club