Based off of the .223 NATO Wikipedia's dimensions photo here.

(I am aware that 5.56 and .223 are different, however, they are dimensionally identical except for the loaded pressure.)

Can be used as a keychain with the loop in the bullet, or in the bottom of the casing.

You can also use PrusaSlicer to add a cut on the keychain model right where the casing meets the bullet to print them in separate colours as I have in the photo. You can also add connectors to easily attach/detach. Could also print the bullet with a ‘tip’ of another colour like red/black/green for different types.

Used the following filaments:

Casing: AMOLEN Silk 1.75mm Gold

Bullet: AMOLEN Silk 1.75mm Red Copper