PLEASE NOTE: this build hasn't been nerfed, but it has been slightly power-crept by a new variant of itself in 1999. The difference between that build and this one is that this one does not rely on a specific helminth ability. You can find the new build here.

Who doesn't want to punish some Grineer with big scary explosive tentacles? :3

In the newest [Jade] Shadows update, Blast was blessed with an incredible rework. Now, when an enemy gets inflicted with 10 blast stacks, a blast with 3x the damage of your average hit affects nearby enemies. Combined with corrosive damage, insane status chance, multishot, fire rate, and beam chaining, primary [Vermisplicer] is an absolute monster at crowd clearing at even the highest level of base Steel Path, and maybe even higher... ;) (i'm yet to try it at level cap) I have since tried it at level cap. It does indeed clear crowds just as well as it does on the Star Chart.

Mod Breakdown:
Rime Rounds // Thermite Rounds - Placed first on the build so that it will combine into Blast. The bread and butter of the build.
High Voltage - Placed last on the build so that it will combine with the innate Toxin and make Corrosive. This partially strips Grineer to allow you to do more damage to them. Combine with a Tau Green Archon Shard for best results.
Galvanized Chamber - Multishot is a flat multiplier to all non-crit non-fr stats on beams. 115 SC at 3 MS makes 345 SC. This SC is multiplied by the MS again at the damage instance, so each status tick does 3x damage as well. A sort of double-dip effect.
Galvanized Aptitude - You have 5 different status effects and ridiculously high status chance. There is no reason not to run this mod.
Primed Shred - You can replace this with any fire rate mod, but the punch-through helps with even better mob clearing. Base Shred is also fine.
Point Strike // Vital Sense - Crit.

Primary Merciless is your only source of base damage. Thus, it can be very hard to ramp up sometimes, but extremely powerful at full stack. That said, unless you have a riven that lets you slot damage somewhere, it's a little bit hard to slot Serration into the build. You can offload the initial damage to Void Strike, Arcane Rage, or perhaps Holster Amp-esque mods. My Saryn has Vigorous Swap for this.

If you want to push this build even further, using it with Saryn has some rather juicy interactions. Blast + Toxic Lash + Roar interact with each other such that Roar quadruple dips into the weapon. With a 60% roar, this boosts your damage by 1.6^4 = 6.5x or 550% in warframe terms. Quite literally, this lets you cut through even Steel Path Sedna Grineer as though they were enemies on base path Earth.

[Vermisplicer] may be currently one of the best Kitguns to use with Blast damage. The other main contenders may be secondary [Sporelacer] or [Gaze] with a similarly thought out build. Though honestly, I think many standard secondaries would outclass them.

Do I recommend this build? For sure. It's quickly become my favourite primary post [Jade] Shadows and I take it to nearly every mission. It chops through all factions with ease and can certainly push even further than I've tested it.