1. Before starting implementing mod code to attachment data you need edit model of sight and add new (or edit one of existing) mesh that will be used when player starts aiming through sight. I highly recommended to add both normal and optimized meshes as new objects as after initial aligning both objects will be in exactly same spot.
If model you using didn't had ADS optimized mesh you need make one from normal sight mesh (Use "Backface culling" option to see what parts of mesh are cluttering field of view and what can be left to preserve shape of sight).

Example of sight setup in blender:
1) Complete model of sight
2) ADS optimized version of sight
3) Basic field of view comparison

After exporting model set optimized objects "enabled" to "false" in object file.

Example:

Code:

    <graphics>
        <graphic_group name="character" enabled="true" culling_object="g_specter">
            <object enabled="true" name="g_lens" inherit_bounding_volume="co_culling"/>
            <object enabled="true" name="g_mount" inherit_bounding_volume="co_culling"/>
            <object enabled="false" name="g_mount_ads" inherit_bounding_volume="co_culling"/>
            <object enabled="true" name="g_reticle" inherit_bounding_volume="co_culling"/>
            <object enabled="true" name="g_visionking" inherit_bounding_volume="co_culling"/>
            <object enabled="false" name="g_visionking_ads" inherit_bounding_volume="co_culling"/>
        </graphic_group>
        <object enabled="false" name="g_specter" />
        <object enabled="false" name="g_glass" />
        <object enabled="false" name="g_gfx" />
        <object enabled="false" name="g_gfx_specter" />
        <object enabled="false" name="g_screen" />
        <object enabled="false" name="g_vr_phong" />
        <object enabled="false" name="g_vr_lens" />
    </graphics>

"g_mount_ads" and "g_visionking_ads" are ADS optimized objects in this example.

2. To implement mod for custom sight you need add object names into 2 new tables: "ads_objects" and "ads_objects_hide". First table defines what objects will be enabled when player starts to aim down sight when 2nd table defines what objects will be hidden.

Example:

Code:

    <WeaponMod id="wpn_fps_upg_o_visionking" based_on="wpn_fps_upg_o_specter" type="sight" texture_switch="false" unit="units/mods/weapons/wpn_fps_upg_o_visionking/wpn_fps_upg_o_visionking" drop="false" texture_bundle_folder="mods" is_a_unlockable="true">
        <stats zoom="6"/>
        <ads_objects>
            <value_node value="g_visionking_ads"/>
            <value_node value="g_mount_ads"/>
        </ads_objects>
        <ads_objects_hide>
            <value_node value="g_visionking"/>
            <value_node value="g_mount"/>
        </ads_objects_hide>
    </WeaponMod>