This is the 10069th most frequent Spanish word.
casquillos
Shells or casings, typically of bullets or cartridges.
In this sentence, 'casquillos' refers to small mechanical or protective components of a frame, such as a part of a window.
Los casquillos de la ventana están oxidándose por la humedad.
The casings of the window are rusting due to humidity.
Here, 'casquillos' is used to describe the spent cartridge cases left after bullets are fired.
En el tiroteo se encontraron casquillos de bala por toda la escena.
In the shooting, bullet shells were found all over the scene.
Within this usage, 'casquillos' represents bushing components in mechanical machinery.