Based off of the 7.62x51 NATO Wikipedia's dimensions photo here.

If you just want the keychain in the photo, it is ‘752x51_keychain.stl’

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

Want a bigger round? Check out my .50 cal version.

Smaller? Try my 5.56/.223 version.