There is currently a Bug in Replication in relation to Weapons/Attachments.

Because of this Bug, attached Sights/Scopes will not Replicated to the Clients if you are the LocalHost.

This only happens if the weapon is placed by WorldEditor and not spawned in a storage.

Example:

If you have a weapon Prefab with attachments and placing this Prefab into the World by WorldEditor, the weapon will be handled different.
If the LocalHost now equip such weapon and changing the attachments like Scope, it will not replicated to Clients (weapons still have the old one attached)

If you try the same, but spawning the weapon inside a Storage, it will work without issue. So i think the Replication problem is placement related.