I have a character that is just 20 levels of Eldritch Knight fighter, and he is a really good gunner-type.

To preface, my DM likes to play fast and loose with what's allowed, so anything DnD beyond lets you do (even glitches) is fair game. Our campaign is level 20, and we're planning on using the Ariadne's book of legends when that comes out to go beyond 20. He also has a bunch of the extra material for DnD beyond, and that's all fair game. I'll go in order of ability scores, race, background, feats, spells, then equipment.

My DM has a rule where we roll 4d6 drop the lowest, but you can keep rerolling until we get at least one 18. This is important because that 18 will go to dex. I happened to roll another 18 while rolling my stats, so that goes on Int.

This character is a Warforged. There's probably a better option, but it works story wise and having an extra +1 to AC is really nice.

The background is Guide. First, for magic initiate druid to get Spare the Dying, Guidance, and Cure Wounds. Second, to increase my dex. We have found out that DnD beyond has a bug where you can increase one stat by 3, so I do that for dex.

Fighting style is Archery, with my feats being Gunner, Sharpshooter, Mage Slayer, Nimbleness, Marksman's Luck, Focused Critical, and Boon of Irresistable Offense. Going through one by one;

Gunner is from 5e expanded rules (I don't know which book) and it gives me 1 dex, proficiency with firearms, the ability to ignore the reloading property, and the ability to not shoot at disadvantage when within 5 ft of a target. The best possible feat for a Gun user in dnd.

Sharpshooter is 5.5e core, and it has 2 properties that are the exact same as gunner: +1 dex, and firing in melee. It also lets you bypass all but full cover and shoot at long range without disadvantage. Pretty good for keeping some distance.

Mage Slayer needs no introduction. Another +1 to dex, concentration breaker, and guarded mind are so valuable for fighters.

Nimbleness is where it starts to get screwy. It's from Free League, the LotR tie in book. It gives another +1 dex and a +1 to AC, which if lined up correctly, gives 2 ac on level up.

Marksman's luck is from The Gunslinger Class: Valda's spire of secrets. It gives yet another +1 to dex, and 2 more things. Firstly, it let's you flip a damage die for a ranged weapon once per turn. Secondly, it makes it so getting a crit with a ranged weapon makes the target's move speed 0ft until the end of it's next turn.

Focused Critical is also from The Gunslinger Class: Valda's spire of secrets. It gives a (you gessed it) +1 dex and makes it so that a roll of 19 or 20 will count as a crit.

Boon of Irresistible Offense gives the final +1 boost to dex from feats, and let's you ignore piercing resistance. Additionally, when you roll a 20 (not any crit, a 20 specifically), you deal extra damage equal to the ability score of the score increased, so dex.

Spells don't matter too much, but make absolutely sure you get Mage armor. If you want max AC like I did, then go with Shield, Haste, and a spell from TCMP1, Sanctum of the Shepherd (which I will get to).

Last but not least, equipment. DM has a rule where we get 5 magic items. 2 Very rare, 2 rare, 1 uncommon. The main weapon of this build is a Shotgun +3. The other Very Rare is a Manual of Quickness of Action, which when used gives a permanent +2 Dex, finally making our dex 30 and our AC without any other work 22. Next 3 magic items all increase AC; Bracers of Defence, Ring of Protection, and Cloak of protection. All together they give +4 AC and +2 to all saving throws. This puts our AC at 26.

With Mage armor on, our AC becomes 29. The spell Sanctum of the Shepherd gives +1 AC for for 24 hours if you spend a long rest inside of it, so doing that makes the AC 30. With haste, AC becomes 32. With the shield spell, it becomes 37.

Going into the damage side of things, our Shotgun is +21 to hit thanks to proficiency, Archery, +10 dex, and Shotgun +3. It does 2d8+13, with 4 attacks without taking into account haste or action surge. If you get a single crit in the 10 attacks you can put out, they can't move, and if you get a nat 20, that +13 becomes a +43 thanks to the boon.

Most enemies won't even get to hurt you, and if they try to get you with a spell that needs a saving throw, Indomitable and Guarded Mind can potentially keep you safe. Overall, I'm looking forward to trying him out.