Borderlands – Does a slag weapon deal more damage

Weapon stability is a hidden stat as it was in Borderlands. Weapon modifiers can improve stability. In fact apparently weapons have "accessories" now (you will see this on the weapon model though you might not recognize it). As with Borderlands, you'll want to take guns out for a spin to see how they handle. I believe can stocks help with stability, so if you see a gun with a stock take it for a whirl. Obviously any scoped weapon will show off stability (or lack thereof), and Sniper Rifles are sort of the kings of this.

Accuracy is a separate stat from stability. I do not know if there is a correlation between the two (and certainly can't speculate on the strength of that correlation).

Some manufacturers were known for making more stable weapons than others though I can't recall off hand who is known for what; keep that in mind. Well, I remember that Tediore makes exploding, teleporting guns.

By the by, this Borderlands 2 Wiki seems the most developed so far. It references stability in several places (including Pixel's link in the comments). It's probably worth pointing out that all of the terms we use may not have the same meanings internally in the engine. Ben, that article about the Accuracy Pool in the comments doesn't seem to contradict what I wrote here, but may well be illustrating how externally we see one model of how things work but it's an abstraction. I don't doubt that stability, recoil, and inherent accuracy interact in strange and mysterious ways in the code!

You are the only one who can add to your challenges, save for one or two specific ones like the car compacter in the dust. So your friends hitting enemies you've slagged doesn't add to the bonus slag damage the challenge has said you've dealt (although it does add to your friends challeneges, since they're the ones dealing the bonus damage), and slag weapons don't deal any bonus damage, only their own damage.

As someone has said you do indeed need to switch to another weapon after slagging an enemy to deal bonus damage. It's also possible to use slag explosive barrels or, since you seem to be playing as the Siren, using the ruin or scorn skills.