Detonator & Slug Shot

This is my first job where I've been concerned with WS mods, so bear with me.

I noticed that for Detonator and Slug Shot the DMG mod is 30% AGI. I was wondering first of all, how is it 30% with nothing else listed?

Furthermore, if the modifier is AGI...why do a lot of people use STR in their Deto/Slug builds?

Obviously I'm not very familiar with WS builds/mods, but I'm here to find out how to pull the most DMG out of my Corsair's WS'. :D

This is what I'm using at the moment, any tips of equip to buy would be appreciated, and yes, I know I need to get an HQ Amemet. >.>