This is a build based around prolonging the duration of the alt-fire through the negative fire rate, and providing CC via radiation and spreading spores on saryn via toxic lash.
The reason for the Magnum force isn't the damage, but the -accuracy to spread out the shots to affect a larger % of the AOE at any given moment.

Be warned tho, this build is only worth the forma if you happen to have gotten a riven like this by chance (not purchase) and feel like meme'ing a bit, as its more lazy than effective when compared to the optimal ways of spreading spores.