I guess the object you instantiate have a NetworkView attached? If so it's quite easy. RPC calls are send to the objects with a NetworkView with the same ViewID.

public class GunInfo : MonoBehaviour
{
    private int m_Bullets = 0;
    public int Bullets
    {
        get { return m_Bullets; }
        set
        {
            m_Bullets = value;
            if (networkView.isMine)
                networkView.RPC("RPC_SetBullets",RPCMode.Others,m_Bullets);
        }
    }

    [RPC]
    void RPC_SetBullets(int aBullets)
    {
        m_Bullets = aBullets;
    }
}

Thanks to the property you don't have to worry about syncing.

If you don't have a NetworkView on your object you have to implement some kind of object management that keeps a list of all objects and using own IDs to find the corresponding objects.

Do you use an authoritative server setup? Or a "normal mixed" setup where clients can create objects? The only important thing regarding my solution is to keep in mind that only the player that created the object can sync the bullet count.