How do the Shadow of the Seraphs relic and the Assassin / Zero's 'Two Fang' skill work with 'burst fire' weapons and each other?

Both the Shadow of the Seraphs relic (introduced in the 'Tiny Tina's Assault on Dragon Keep' DLC) and the Assassin / Zer0's Tw0 Fang skill have the following effect: "Every time you fire a gun you have a chance to fire twice".

What if I use either with a gun that has a burst fire effect? For example, Sledge's Shotgun - special weapon effect description: "Burst-fires two shots per trigger pull, with a long delay between each burst. Reduced clip size".

How many shots will fire per trigger pull if the Shadow of the Seraphs' or the Tw0 Fang's "fire twice" chance activates with a 'burst fire' weapon?

Also, what if I am playing as Zer0 and have both the Shadow of the Seraphs relic and at least a point in the Tw0 Fang skill? How will their chances of firing a gun twice stack?

The idea of Zer0's "tw0 fang" skill is to improve Zer0's designed game play, the way he was supposed to be played. Zer0 is fantastic as melee, sniping and pistols above all when containing under-barrel blades so this skill is to benefit single shot weapons over all others. in short, there is NO effect on burst fire weapons that can help you, the only possibility is that the first two shots of a burst would be fired in a quicker succession, some players may find that helpful to speed up the moment as I am sure we have all had those moments in Borderlands where you wish you could of got that one extra shot on the enemy in second wind.

As for the the relic, again this has the same effect and although some player may find it unworthy of use, I would personally state that no harm can come from this feature, only positives ... unless you are extremely O.C.D and demand the firing sequence of your gun to be fired perfectly in sync with one-another. as for stacking up the effect, Yes. This will improve the statistics already based on Zer0 and will give you grater chance of achieving those extra shots!

Hope this helps, enjoy your massacre!