Why is it fine for shotgun shells to be made of plastic but polymer casings for rifle cartridges are considered “unreliable” or “bad” ?

Question

I’ve never fired/handled guns but have some knowledge of guns and their basic mechanisms, and this thought just came across. If you could make cartridges out of polymer/plastic, and they are significantly lighter, wouldn’t you want to switch to them asap? yet there is no adoption. On the other hand shotgun shells have always been plastic but it’s not problematic in any way? why is this the case?

In: 24

Answer 1

A 12 gauge shells operate at about 11-14 thousand psi. The 5.56mm NATO operate at 62 thousand PSI.

But your assumption is partially wrong to begin with. See the part of the shotgun shell where the power explode is actually made of brass, the same material as a rifle casing. Different shotgun shells will have a different length of brass (low vs high brass) that depend on how much powder the shell will be loaded with.

The plastic part of the shell is not where the powder is, but where the pellets are. The equivalent in a rifle cartridge would be where the bullet is, which have nothing around it. The plastic is only there to keep the pellets together.