The shot came from his right, which appears to be an entrance into a room to the south. There's a corridor to the left (north).

Mr.Dark ducks into cover and glances at the group, “Less than friendly reception. I’m going to check behind us.”  
He moves back to the mouth of the cave, trying to stay hidden, and peers out seeing if anything is trying to approach from behind. If he sees nothing he returns to the group, “We’re clear. Who is the first in. Want me to go lead?”  

     "That is gnome way to treat guests!" Kaemgen shouts out, striding purposefully towards the entrance to the right.  "We are friendly visitors!  We are here on a mission from the mayor of Phandelin themselves!  Stop this attack right now or I shall show gnome mercy!!!"

     Kaemgen reaches down and grabs his weapons as he approaches the door, Battleaxe in his right hand and Warhammer in his left.

Mr.Dark nods with respect to Kaemgan's back as he strides forward, "Or you could lead us in."  He follows behind Kaemgen in a crouch, his crossbow out, muttering a quick prayer to Tymora to let her luck keep the enemies blows away. (Shield of Faith) 

Auric:

”This would seem a churlish reception, though mayhaps the attack was impersonal—I have heard tales of fantastic gnomish devices.  Perhaps one such is purposed for the defense of their home,” allows Auric graciously, but he follows carefully nonetheless, much more concerned now.

Kaemgen bravely armed with puns and his guns, he finishes his entrance in time to see a rotating platform equipped with four heavy crossbows finish clicking into place. Mounted above the crossbows at a height of 6 feet is a chair equipped with pedals that causes the entire contraption to spin counterclockwise, and with levers that reload and fire the crossbows. Sitting atop is A wide eyed gnome with two poofy buns that stick out from each side of her bandanna. She grins with giddy glee at the sight of the man announcing himself with such enthusiasm.

Facktoré
(I'm thinking a Louise from Bob's Burgers with this one)

Over the roar of the echoe of the waterfall, Kaemgen catches the sound of the autoloading feature of this crossbow unit just finishing it's last motion. He couldn't help but get the looming feeling that Auric's words were the truth. The gnominventor would cackle. Click.

Crossbow Bolt
Attack: 8 Damage: 7 

Map Update:
(I worked on this, I better post it! >.<)

      "Stop!" Kaemgen would shout, rushig up and attacking the contraption, hopefully smashing it before it could rotate and shoot again.

Warhammer vs Contraption Attack: 13 Damage: 7

BattleAxe bonus action vs Contraption Attack: 21 Damage: 11

     "Stop it now!" Kaemgen shouts out again, kind of mindless of whether the contraption had been stopped yet or not, and uses his Action Surge to swing one more time...

Action Surge Warhammer vs Contraption Attack: 25 Damage: 7

Mr Dark runs into the room then strafes to the left, keeping his back to the wall. He fires a couple of bolts into the machinery hoping to gum up the works. 

Attack: 15 Damage: 9

Attack: 9 Damage: 6

plus 5  sneak attack damage to either that hits. 

Auric:

Moving to where he can see the ruckus from the hallway to the north, Auric roars, “Stop thine assault, Gnome, before I burn thy prized contraption to cinders!”

Intimidation:  19

Kaemgen rushes in, swinging. He made himself more of a presence than a threat with his first swings. In his defense, they were new weapons and he needed to get a feel for the weight of them in his hands. By the third swing, his hammer knocked a joint loose! Or something. He hit something. But it still looked functional.

That was until Mr. Dark swoops in during the commotion. His first both easily dismembers the first crossbolt potion Kaemgen was hacking at. Finding a sweet spot, the sniper shows this contraption how to properly use a crossbow. As if guided by lady luck herself, the shot fires with such accuracy it ricochets within the gnomish system, completely dismembering two of the mechanisms with one bolt.

Then in comes in a shiny imposing dragon with a booming voice that echoed off the walls just right. The gnome sat there flabbergasted. She was completely stunned. After a moment, she'd concede, crossing her arms and slouching slow within her seat. She'd force a scoff and mutter under her breath to herself in gnomish. Looking down like a scorned child, she'd peak back up with a hint of a pout.

When someone would approach her, she'd say casually. "Hi, my name’s Facktoré." She'd pause briefly to dig into her pocket and then she'd hold out her hand. "I would like to donate a piece of my personal chalk." The gnome would insist before looking over to another in the group. "In case you need to outline a body."

Auric:

Still incensed, Auric moves forward and says, "Mayhaps I was too optimistic about thy intentions, Factore, when I assured my companions that the gnomes would not be so knavish as to attack those of us who should be welcomed as honored guests.  Our intrepid company entered peacefully and hailed thee with high honor, but you fired on us as enemies.  Could this be how the gnomes treat their allies?  I had not previously numbered thy kind among the evil races, but thy hostility has given me pause."

Facktoré would nod along  with the dragonborn. Mmmhmm. Mmhmm. She'd wait for him to say his peace. "Yeah well, let's see how you react when you've got an impostor in your midst!" Her eyes expand in jest. "I'll have you know thine sir that I have direct orders from the Kings to ensure m- the safety of others!" Telling the truth, and still feeling as though she's done nothing wrong, the gnome perked with pride. "No harm. No foul."

"Impostor? You got some kinda shapeshifter in your midst?"

“What do you mean an impostor?” Scott’s voice can be heard from his position in the hallway. Not wanting to risk being tagged with another crossbow bolt. 

"There was harm you fool. You shot our friend and now he's bleeding all over the hallway back there. You moronic gnome. Glad we broke your stupid machine."  Almost seething with anger, Mr.Dark keeps a bolt loaded on his crossbow. He takes a few heated deep breaths to calm himself down. "Scott, come on in, the machine's broken.  Yeah gnome, what's this about an imposter?" 

Mmmhmm! Mmmhmm! Facktoré would nod along with the blue dragonman and the other guy. She was just about to respond to them with the big angry guy comes up with insults. Her eyes would narrow a little at him, then she'd lean over a moment to take another look at 'the other guy'. Wouldya look at that, she did hit him. "Did I do that?" She'd blurt before shrugging. "He's fiiiiiiinneee. Right? See! He's fine. Barely even a peep." The engineer would nod. Ignoring meany man who brought name calling to the discussion, she'd look to the original one's who asked the question. "Welp! The King's been yammering about some imposter 'nomming' gnomes. Locked himself and his hubby in their bedchamber." She'd sit back comfortably in her contraption with her hands behind her head and puts her feet up on the dash. "And I'm the only one brave enough to face anything in these here tunnels. Needless to say, I checked you guys out, and you haven't tried biting my head off... Okay well he's pretty sus..." She points to Dark, then to Auric with a 'so so' gesture. "But like, the rest of you... y'alright." Facktoré would nod.

Mr.Dark shakes his head, "Nomming gnomes? Sounds like a bad joke. Is this a real thing?" 

     Kaemgen eyes the contraption suspiciously and gives it another poke with his Warhammer.  “So you’re shooting at anything and everything because someone is impersonating gnomes?” He asks, skeptically.  “Aren’t we a little tall to be gnome troopers?”

     “But we pass your muster now, yes?  Perhaps we would have your leave to go talk to your kings, as the mayor of Phandalin has requested us to do?  We have dire warnings…” Kaemgen continues.  “But perhaps could look assisting in making sure gnome more gnomes get nommed…”

Auric:

As the conversation progresses, Auric moves over to check on Marblebeard.  Taking out his orb, he coaxed a soft glow from it and lessens the extent of the injury.

Healing Light:  1

Addressing the gnome, he says, “Very well, thine overzealous  approach to defense may be understandable given the deceptive nature of your foe.  Couldst thou describe this imposter creature further, and procure for us an audience with your king?”

Persuasion:  23

Auric:

(Another die of healing:  5 )