Can an asset as small as a bullet travel across server shards?
TL;DRNo, assets can't cross shards unless stowed in an inventory first, and within a shard, entities like missiles can span server nodes while bullets are spawned separately on each client and node.
The short answer is no.
You can see shards as a completely isolated instance of the simulated universe, very similar to how we currently have different isolated instances per dedicated server. In order for items to transfer between instances, these items need to be stowed into an inventory before they can be unstowed into a different shard. For example, if a player picks up a gun in one shard then places that gun into their backpack. Now, when the player connects to a different shard, the player can take the gun out of their backpack, unstowing it into the new shard.
Within a shard, an entity like a missile will be able to travel across multiple server nodes if these server nodes have the missile within the server's streaming area. Only one server node will be in control (has authority) over that missile, while the other server nodes will only see a client view of the same missile.
Bullets are actually spawned client-side. So, a unique version of the bullet is spawned on each client and server node, which is why I used a network-replicated entity like a missile in the example above.