Solid brass rivets often used with matching brass washers for an old school method of joining components using the peining face of a ball/pein hammer.

Note: By international convention many fasteners - bolts, set screws and rivets etc - are sized nominally.

If a 6mm rivet was in fact 6mm diameter it would likely be an interference fit in a 6mm hole. Bolts,set screws, ribets and the like are made slightly undersize to offer a clearance fit..   

See HERE for a conversion table