>Rimfire ammo is limited to smaller calibers since the cartridge walls need to be thin enough to be able to be crushed by the firing pin and ignite the primer.

> The downside?

> Well, the nature of the casing means it’s pretty much limited to small calibers. You have to have some relatively flimsy brass to handle the rimfire set up. As a result, the powder necessary to propel a larger bullet would blow the brass apart.[1]

Correct about the buffer tube. So even if you installed a folding stock on an AR-15, it require some proprietary buffer tube to work like this one.[2]

[1] https://www.pewpewtactical.com/rimfire-vs-centerfire-ammunit...

[2] https://youtu.be/_qr2GTq3v3w