Wear and weak are not the same fyi.
It does both, steel ammo hits for about 5% less damage, pistol for example is 25 with the brass casings for damage, while the steel ones are 24 damage and double degradation rate. You notice it more on the higher damage bullets more.
Base Magnum Bullet
<item name="ammo44MagnumBullet">
<property name="Extends" value="ammo9mmBullet"/>
<property name="EconomicValue" value="19"/>
<effect_group name="Base Effects" tiered="false">
<passive_effect name="EntityDamage" operation="base_set" value="80"/>
<passive_effect name="BlockDamage" operation="base_set" value="12"/>
</effect_group>
</item>
Steel version.
<item name="ammo44MagnumBulletSteel">
<property name="Extends" value="ammo44MagnumBullet"/>
<property name="CustomIcon" value="ammo44MagnumBullet"/><property name="CustomIconTint" value="a0a0ff"/>
<property name="DisplayType" value="ammoBulletSteel"/>
<property name="Material" value="Msteel"/>
<property name="EconomicValue" value="14"/>
<property name="DescriptionKey" value="ammoSteelCasedGroupDesc"/>
<effect_group name="Base Effects" tiered="false">
<passive_effect name="EntityDamage" operation="base_set" value="76"/>
<passive_effect name="BlockDamage" operation="base_set" value="10"/>
<passive_effect name="DegradationPerUse" operation="perc_add" value="1"/>
</effect_group>
</item>
The steel bullets are there solely in the case you gun out of brass, the only diff is the casing is made with iron insted of brass.