Thanks for the clarification. I wanted to be helpful, but knew I was mostly guessing. It is interesting that it pulls from the general stack size and can't be modified separately.
It's because of what it defines as the ammo for it:
<property name="AmmoItem" value="ammo9mmBulletBall"/>
After helping the OP out, I got to thinking about it and now working on some code to make it an unique stack number compared to what you carry in your inventory. I think if you were to make a new item, say
<property name="AmmoItem" value="ammoTurretMagazine"/>
and create a recipe for it also, you can have an unique item for the turret that takes a little of effort to do.
This is the code I got so far, but haven't tested yet
<insertAfter xpath="/items/item[@name='ammoBundleCrossbowBoltExploding']">
<item name="ammoTurretMagazine">
<property name="Extends" value="ammo9mmBulletBall"/>
<property name="ItemTypeIcon" value="bundle"/>
<property name="Material" value="Mbrass"/>
<property name="Weight" value="0"/>
<property name="Stacknumber" value="1000"/> <!-- STK ammo high -->
<property name="EconomicValue" value="0"/>
</item>
</insertAfter>
<insertAfter xpath="/items/item[@name='ammoBundleCrossbowBoltStone']">
<recipe name="ammoTurretMagazine" count="1" craft_time="1000" craft_area="workbench" tags="workbenchCrafting">
<ingredient name="ammo9mmBulletBall" count="1000"/>
<ingredient name="modGunDrumMagazineExtender" count="1">
</recipe>
</insertAfter>
This is what I whipped up last week, but I want to make some other changes (like have the Int tree affect the damage output and crafting times).