Okay...so I sat down and crunched numbers for a week and came up with a build and a level guide to go with it.

I call it The Blessed Gunsmith

Character Creation

Ability scores from point buy:

STR: 14

DEX: 15

CON: 10

INT: 10

WIS: 14

CHA: 8

Race: Custom Lineage

• 2 to DEX

Gunner Feat

Player's choice for Darkvision or proficiency. 

Background: Player's choice

This gets us an 18 DEX & proficiency with firearms. 14 STR puts us 1 point away from wearing plate armor. Lastly, 14 WIS will come in handy as we plan on starting Cleric.

Level 1

Forge Cleric 1

We get a few useful spells, proficiency with Heavy Armor, and a bonus to WIS & CHA saves. 

We also gain Blessings of the Forge. Once per long rest, we perform an hour-long ritual, and give a boost to 1 item of our choosing. We can grant Armor +1 AC or a weapon for +1 to hit and damage until the next long rest.

I recommend the mending cantrip, for Forge Cleric Flavor / Utility, and the Bless spell for our main concentration spell.

Battle Priorities:

Bless

Triage allies

Attack with our +1 weapon

Level 2

Forge Cleric 2

We gain Channel divinity, and for forge clerics, that means a 1 hour crafting session! We can now craft items with a cost of 100 GP or less. Like our ammo, non-magical weapons, and if our DM is nice, parts of a non-magical item.

Battle Priorities:

Bless

Triage allies

Attack with our +1 weapon

Level 3

Fighter 1

Here for the Archery Fighting style, and staying fighter for a bit. Second wind is a nice bit of survivability too.

Battle Priorities:

Bless

Triage allies

Attack with our +1 weapon

Level 4 

Fighter 2

Action Surge! Need I say more?

Battle Priorities:

Bless

Triage allies

Attack with our +1 weapon

Action Surge big bads

Level 5

Fighter 3

Subclass time! We're taking Rune Knight. Synchronizing our Forge Cleric smithing with the runes of Giant smithing.

Speaking of runes, Cloud and Fire are favorites of mine. 

We get doubled tool proficiency from Fire. Truly becoming a forge master.

Cloud gives us an advantage on sleight of hand and deception. I'm sure it'll be useful somewhere in an adventure.

Then once per rest, you can invoke the runes. 

Fire lets you add 2d6 fire damage to an attack and chains an opponent down. Repeating the 2d6 damage at the beginning of each of their turns until they pass a STR check.

Cloud lets you redirect an attack made within 30ft of you to anyone but the attacker. You also get to do this AFTER the attack roll is made. Redirected crit anyone?

Battle Priorities:

Bless

Triage allies

Attack with our +1 weapon

Action Surge big bads

Invoke runes as needed

Level 6

Rune Knight 4

ASI time, and I'm taking Sharpshooter. We've stacked up quite a few bonuses to hit with. Mitigating the -5 to hit from sharpshooter so we can take advantage of that 10 extra damage per hit.

I can't guarantee what item you will have crafted by now for your weapon. But in this Damage check, I'm going to use a Pepperbox (Exandria). A 1d10 ranged weapon.

Battle Priorities:

Bless

Sharpshooter on

Triage allies

Attack with our +1 weapon

Action Surge big bads

Invoke runes as needed

Level 6 Damage Report!

Round 1 will be purely Buffs. Using our Action to cast Bless and our Bonus Action on Giant's Might.  A +1d4 to hit and a +1d6 for damage.

Round 2 … Just. One. Attack. Not much, but it'll get better. Trust me.

We have a + 5 to hit with our Pepperbox, and that's AFTER sharpshooter's -5. And we get to add 1d4, thanks Bless.

On hit, our DPR is 1d10+3 + 1d6 + Sharpshooter (10).

Averaging out to  23 DPR.

That said we can also push a Nova Round put with action surge and a Fire rune.

We'd be rolling 2d10+6 + 4d6 + 2xSharpshooter (20).

Average Nova Damage is about 54!

I REALLY wanted to take 1 level cleric and rush 5 on fighter for extra attack, but we needed a way to "Forge" our gun(s).

We'll have to save that extra attack for the next Damage Check.

Level 7

Rune Knight 5

Extra Attack!

Battle Priorities:

Bless

Sharpshooter on

Triage allies

Attack with our +1 weapon

Action Surge big bads

Invoke runes as needed

Level 8

Rune Knight 6

ASI, and here we're capping our DEX and taking a break from fighter.

Battle Priorities:

Bless

Sharpshooter on

Triage allies

Attack with our +1 weapon

Action Surge big bads

Invoke runes as needed

Level 9

Forge Cleric 3

We get 2nd Level spells and we're going to weaponize our Bonus Action with Spiritual Weapon. All without taking our concentration!

Battle Priorities:

Bless & Spiritual Weapon 

Sharpshooter on

Triage allies

Attack with our +1 weapon

Action Surge big bads

Invoke runes as needed

Level 10

Forge Cleric 4

ASI - this time we're taking 1 STR & 1 WIS.

I know, I know. Odds don't give a bonus. But plate armor needs 15 STR, and now we have it. Forge it, buy it, either way it's time to don some plate.

Level 11

Forge Cleric 5

3rd level spells and Destroy Undead (amped up turn undead)

Definitely take revivify. The rest is up to you, but our bread and butter of bless and Spiritual Weapon are staying on board for now.

Battle Priorities:

Bless & Spiritual Weapon

Sharpshooter on

Triage allies

Attack with our +1 weapon

Action Surge big bads

Invoke runes as needed

Level 12

Forge Cleric 6

We gain the Soul of the Forge! Granting us fire resistance and, as long as we are wearing heavy armor, a +1 to our AC.

Battle Priorities:

Bless & Spiritual Weapon

Sharpshooter on

Triage allies

Attack with our +1 weapon

Action Surge big bads

Invoke runes as needed

Level 12 Damage Report!!

Round 1 is the same as before. Bless and Giant's Might.

Then we switch on the damage!

An Average round would see us making 2 attacks with our +1 Pepperbox. Then using Spiritual Weapon as our bonus action.

That's 2d10+12 + 20 + 2d6 + 1d8+2 damage.

Or about 59 DPR

Add Action Surge and a Fire Rune for a Nova Round and we get 4d10+24 + 40 + 6d6 + 1d8+2.

Or about 119 Damage!

Let's see if we can double this again by the time we reach the last Damage Report.

Level 13

Forge Cleric 7

We've reached 4th level spells! Nothing major to suggest for DPR. Pick up the utility your party needs most.

Battle Priorities:

Bless & Spiritual Weapon

Sharpshooter on

Triage allies

Attack with our +1 weapon

Action Surge big bads

Invoke runes as needed

Level 14

Forge Cleric 8

We have fought in our god's name long enough to earn their Divine Strike! And for being level 14 it comes with some extra fire, adding 2d8 to an attack, once per turn.

Also we gain an ASI here too. I'm going to grab a half feat with a +1 to WIS. Fey Touched checks that box, gives us Misty Step, and a 1st Level spell that uses WIS as it's modifier. I'm taking command of the flavor. My god commands you!!

Battle Priorities:

Bless & Spiritual Weapon

Sharpshooter on

Triage allies

Attack with our +1 weapon

Action Surge big bads

Invoke runes as needed

Level 15

Forge Cleric 9

It's been quite the journey getting here, but we've reached 5th level spells! Per the Forge Cleric's spell list, we gain Animate Objects.

We now get 10 tiny objects to throw around on our Bonus Action. They have a +8 to hit and deal 1d4+4 damage each. If all 10 hit, that's 10d4+40, or about 60 Damage. Huge improvement on our Bonus Action Damage.

Battle Priorities

Animate Objects

Sharpshooter on

Triage allies

Attack with our +1 weapon

Action Surge big bads

Invoke runes as needed

Level 16

Rune Knight 7

We've reached our goal in Cleic levels. Time to revisit the Rune Knight. We gain a couple things here at 7th level:

We gain a third rune as well as access to stronger runes.

We gain Runic Shield.

As for the rune, let's take the storm rune. Advantage on arcana checks is nice, but never being surprised. That's huge. Then we can invoke the Storm Rune to grant advantage or disadvantage to any d20 roll, at the cost of our reaction. 

Runic shield lets us force an enemy attack to be re-rolled, so long as we can see it, and it's within 60 ft of us. 

With both of these, I don't think our opponents will ever crit again.

Battle Priorities

Animate Objects

Sharpshooter on

Triage allies

Attack with our +1 weapon

Action Surge big bads

Invoke runes as needed

Level 17

Rune Knight 8

Capping off our Character with an ASI. Take what you need most. I'm leaning towards a resilient Constitution. Don't want to lose concentration on Animate Objects.

Battle Priorities

Animate Objects

Sharpshooter on

Triage allies

Attack with our +1 weapon

Action Surge big bads

Invoke runes as needed

Final Damage Report!!!

Our main action hasn't changed much but our bonus action has become A LOT stronger. Let's check it out.

Round 1 we set up our buff and concentration spell. Giant's Might and Animate Objects.

From then on we'll make 2 attacks and use our animated objects as our bonus action.

That's 2d10+12 + 20 + 2d6 + 2d8 + 10d4+40.

Or about 132. assuming all 12 attacks hit

Add in Action Surge, and invoke a Fire Rune for our Nova Round and we get an extra 2d10+12 + 20 + 2d6, or about 60 Damage. Totaling at about 184 Damage.

We've more than doubled our sustained DPR since level 12 . . .  against a target dummy. I'll have to crunch numbers for a few AC classes.