What did you use? Alloy, flux? The casing is a brass, so it should be rather easy to work with a silver braze. Or you can use a regular soft solder if the loop is anchored on the other side.
If it doesn’t work, mechanically clean the joint area to bare shining metal and use a more aggressive flux. (Then use ultrasonic eyeglass cleaner to get rid of the flux residues.)
More about the brasses used e.g. here:
There are also bullet jewels with hated one’s name on it. I made one on an order, for a friend, from a spent 9mm shell, with engraved name of her ex. (No, he did not get shot.) Didn’t look half bad. No loop though, just a pair of 2mm (2.5mm? Don’t remember) holes at the base and thin leather strip pulled through.