Level 6 Gun Fighter Optimization

Question

The setting: The setting is Brancalonia, an E6-like low magic 5e setting. Gold is low, magic items are low. Long rests take a week. The setting is modeled after Italian spaghetti fantasy. Levels are capped at Level 6, although there is some ability at level 6 to still grow, allowing picking up to a max of 2 feats or 2 ASIs instead of gaining levels once you reach 6. Challenge ratings can go higher than 6. Magic items are EXTREMELY rare.

The character: A level 5 ratfolk fighter (Samurai). Small size, 25ft move, stat distribution:

Stat	Score
Str	10
Dex	17
Con	16
Int	10
Wis	14
Cha	8

Equipment: Rifle, 1d12 Piercing, Range 60/150; Sentinel Shield; Moontouched Rapier (for overcoming damage resist); Cloak of Extravaganza (18-20 crit range for everything, 1-3 failure range for everything, attuned); Broom of Flying; Studded Leather armor. Feats: Gunner.

Strategy: As I'm small, I'm restricted to weapons without Heavy, making firearms in the setting the strongest range attack I can. 1d12 turns into 2d12 when I crit, hence the triple crit range item. My purpose in the party is consistent range damage, and off-tanking as necessary with the rapier and shield. AC is 15 without shield, 17 with. My general combat actions are to hide or use the samurai ability, then unload with attacks from range. As needed, sword and shield to protect the squishier party members, as the fighter HD and +3 con gives a good chunk of HP to soak.

Question: Given I only have one more level of growth, which is either an ASI or a feat, and likely only one ASI or feat for "growth" past level 6, I'm trying to optimize my damage output, while also taking into consideration that I need to avoid rolling a 1 at all costs. Why? Firearms rules in Brancalonia have a special caveat for rolling a 1 on attacks where a misfire happens. Rolling poorly on this misfire table can do anything from the gun backfiring, dealing me fire damage, to the gun flat-out exploding, destroying it. Note that although my failure range is increased 1-3 by my cloak, the misfire still only happens on a 1 - the DM went and got a ruling from the setting creators themselves.

Given this gun cost me 250 gold in a setting that an entire session can give you 20 gold at max, I have invested a large chunk of money into my firearm, and it is my primary damage dealing tool. Its loss would result in my having to go back to a light crossbow, greatly decreasing my damage output. I have to have some way to avoid rolling a 1 on attacks at all costs. My go-to here is the Samurai ability (all attacks this round have advantage), only attacking when I have advantage (from hiding), or using Inspiration to reroll a 1, whenever I have it (Inspiration in the setting works to let you reroll after the fact, rather than before, as is RAW).

What's my best level up with the assumption I only get a combination of two feats or ASIs?

My thought process is that the next feat (Level 6 Fighter) should be Piercer. +1 Dex, another die of crit (Bringing my total crit damage to 3d12), and one/turn damage dice reroll. My gun deals piercing, crossbows deal piercing, rapiers deal piercing. All of my damage is Piercing, although non-magical.

The question of what I do with that second 'level' is much more difficult. I could either ASI Dex 18->20, take Sharpshooter (making my range 150 instead of 60, allowing me the sharpshooter attack to deal 10 more damage for -5 to hit), or take Lucky (allowing me 3 rerolls a long rest to avoid the possibility of the gun exploding on a 1, by avoiding the 1). The ASI and sharpshooter are both damage increases, while Lucky mitigates the disaster scenario, functionally giving me three inspiration for each quest. Sharpshooter means I ignore cover and have longer range, but how often do ranges above 60ft happen? Is 3d12 enough crit damage, or could I increase this more? Should I get something other than Piercer? Are there any feats I've missed? Is the +2 dex from the ASI my best option? Am I missing some magic item that would help my situation greatly?

Answer 1

Lucky

If you're really worried about rolling a 1 because of the risk of your gun just exploding on you, you should prioritize taking Lucky. That's one of very few ways to change your result after you already know you rolled a 1.

For the record, Inspiration does not work the way you think it does. Inspiration has to be spent ahead of time, not after you roll. It's possible your DM is using a house-rule on this, but officially:

If you have inspiration, you can expend it when you make an attack roll, saving throw, or ability check. Spending your inspiration gives you advantage on that roll.

Note that it lacks the special language present in Lucky:

Whenever you make an attack roll, an ability check, or a saving throw, you can spend one luck point to roll an additional d20. You can choose to spend one of your luck points after you roll the die, but before the outcome is determined.

(That language is found in a few other rare places, but not in Inspiration.)

Ability Score Increase

After Lucky, using an ASI to bring your Dex up to the max of 20 is your next priority. Dex is the most overloaded skill in the game — it potentially impacts AC, attack roll, damage, and a several valuable skills/tools.

Sharpshooter

As far as Sharpshooter goes, you said:

My purpose in the party is consistent range damage

If that's the case, I can't recommend using Sharpshooter. The -5 hit/+10 damage is mathematically a good deal that increases your expected damage-per-turn, but it makes you much more "swingy" — you're a lot more likely to roll a total miss, which on average is counteracted by hitting for massive damage when you connect — which is the opposite of consistent damage. To some extent, it also fights with your crit range expansion — there's a risk that with the -5 in place, a rolled 18 becomes effectively a 13, and that might be a miss against a more heavily armored enemy. (Ideally, you'd skip the -5 when firing on heavily armored targets or weak targets that are likely to die from one hit anyway, but just to say it.)