This lead to me thinking, for quite a long time, that gun turrets could not use piercing rounds. This became... problematic... when defending against more evolved biters. It was only after noticing a post on the forums saying that they CAN use piercing rounds that I realized that you need to physically remove the normal ammo before you can put the piercing ammo in.
Hilariously, this bug happens in the other direction, too - if you try to swap piercing bullets for regular bullets the game claims that regular bullets can't be used as ammo for gun turrets.
![Smile :)](./images/smilies/icon_e_smile.gif)
![Image](https://forums.factorio.com/images/ext/2ec03a628a858f28510673de6feab7bf.png)