Your build is pretty solid. I would assume you are using a Leg.Killer with this.
Shadow of the Seraphs is cool but you will tend to get more out of a Bone if you are using elemental shotguns.
Regarding B0re, I personally wouldn’t strategize my shotgun usage upon their B0re efficacy. In mobbing, the circumstances where B0re is gamebreaking is far and few between, and many of the enemies with overlapping hitboxes tend to be bosses anyway.
Blockhead is an excellent choice. While it’s DPS is inferior to Zer0’s top options, its splash and spread lend it alot of versatility while mobbing. It is a good balance between a OSOK shottie and a DPS shottie.
As many have already mentioned, the Quad, and especially a Rustler’ Quad, is not great on Zer0. If you are a fan of Jakobs, the vanilla Coach Gun would serve you best. If you are utilizing a Leg Killer, I highly recommend pairing it with a Jakob’s Relic so you can take advantage of its FR by having 4 shots in a mag.
Outside of the Peak, I wouldn’t really recommend the Twister. You would be better served by most triple and quad barreled shotties that can consistently score critical hits.
If you are aiming for damage-per-shot, taking advantage of OSOK and Decepti0n, I would recommend the Jolly Roger as your best choice. It’s very underrated around these parts, but I have not found any other shottie more capable of consistently scoring one-shots on Zer0. The Shotgun Supreme is also an excellent choice.
For DPS, I find the Thinking and Heartbreaker to be excellent. If you are good at getting the sweetspot, the Interfacer would be superior, but this is quite difficult to achieve in bandit maps. Hyperion shotguns may not have amazing per-shot damage, but their DPS is pretty insane, giving it excellent synergy with Rising Sh0t. Also, they tend to be very accurate - and can take advantage of Zer0’s crit buffs well.
The Coach Gun and Blockhead are also really solid - giving a good balance between damage-per-shot and damage-per-second.
I’m sure @DeputyChuck would have something to add, given this is his favorite topic.