Does the drum mag work for the guns?
To add onto Izayo's reply, the way to add (or remove) the item modifiers is based on the tags system:
<item_modifier name="modGunMagazineExtender" installable_tags="magazine,turretRanged" modifier_tags="magazine" blocked_tags="noMods" type="attachment">
<item_modifier name="modGunDrumMagazineExtender" installable_tags="drumMagazine" modifier_tags="magazine" blocked_tags="noMods" type="attachment">
So based on the tags, the clip extender mod can apply to any item that has magazine or turretRanged in its tags, while the drum extender can only apply to drumMagazine
<item name="gunHandgunT3DesertVulture">
<property name="Tags" value="weapon,ranged,revolver,holdBreathAiming,reloadPenalty,gun,shortRange,pistol,barrelAttachments,sideAttachments,smallTopAttachments,magazine,firingMode,attAgility,perkGunslinger,attachmentsIncluded,canHaveCosmetic,handgunSkill"/>
<item name="gunHandgunT3SMG5">
<property name="Tags" value="weapon,ranged,holdBreathAiming,reloadPenalty,gun,shortRange,barrelAttachments,sideAttachments,smallTopAttachments,magazine,drumMagazine,firingMode,bottomAttachments,attAgility,perkGunslinger,9mmGun,attachmentsIncluded,canHaveCosmetic,handgunSkill"/>
So based on these two examples, the Desert Vulture can use the the clip extender only while the SMG can use either.
Assuming this code still works, you can add the capability of adding the drum magazine to the Desert Vulture by
Code:
<append xpath="/items/item[@name='gunHandgunT3DesertVulture']/property[@name='Tags']/@value">,drumMagazine</append>