I also got the III from a drop. What do you like out of explosive/piercing ?
Either seem to kill or hit lock enemies
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The other 2 upgrades are stability and lightweight, so Explosive rounds is the best choice. If I had AP I would use that, And XR works well on a Kill (head) shot.
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This random drop has Armor piercing / snapshot / explosive. I use armor piercing as I more often have enemies standing in a line than close enough to take a lot of explosive damage.