Still broken in the sense of being under-defined, this time probably underpowered

It's gonna be hard to give a full analysis of the things in the class. In part because at some point you just have to playtest and see, but also because there here are overshadowing issues elsewhere. There's gonna be an element of just going through explaining the things which have problems or don't work.

Gunsmithing

And I don't think we're off to a good start. This is a crafting feature, which is maybe a natural way to model person-who-makes-weapons, but it has some severe problems. Very simply, this probably does nothing at first level. Possibly never. Crafting is a downtime activity, and the availability of downtime is dependent on the campaign. Some might be laden with it, in others you might never have any. That's not a good thing for your sole first level feature to be dependent on. A similar note applies to the gold cost, though expecting a character to get gold over the course of a campaign is a lot more reasonable.

Recommendation: assuming you're going for is customizable, bespoke weapons crafted by the Gunslinger, do something modelled off the Artificer's infusions. Whether to make it as fiddly as the modifiers currently incorporated by reference is up to you.

Base weapons

If we accept that we basically don't have a first level feature (or only depending on the campaign), at early levels you basically have a vanilla Ranger (and I hope you're familiar with the class's reputation). Except you have a smaller hit die, and the weapons you get are probably worse. The probably is in part because ranges are poorly defined, but that's not gonna matter too much.

The rifle and pistol are probably the easiest comparisons. While the rifle has a d10 instead of d8 damage die, you only get 2 attacks per 3 actions (and that rate gets worse once you get Extra Attack). The pistol is par on damage, except it eats up your bonus actions, and that's gonna start mattering later (also later you get Extra Attack, which pistols work poorly with).

In general though, the design with N capacity and whole actions to load is gonna be difficult to design around. You're gonna have a hard time making sustained damage be level with other classes, and otherwise you might well get more 1/encounter powers (and reload between combats/as part of a short rest) but if they are that you should design them to be that and match up to other classes variants thereof, and I think the current design falls short on both fronts. I'm not going to offer specifics on the guns that aren't starting equipment, because how and when they are acquirable isn't well enough defined for them to be evaluated, but I think there are some effects which could quickly be very problematic.

Shooting styles

Similar to Rangers you get Fighting Shooting styles at 2nd level. Of note, Rangers also get spellcasting at that level, so now you're definitely into worse than Rangers territory. While as a general design these are fine (unsurprising as they are, wisely, modelled off fighting styles), but show some issues from the under defined nature of some of the material. In order:

• Dual-Wielding is generally fine, except none of the weapons have the light property, meaning two-weapon fighting isn't actually possible out of the box.
• Explosive doesn't have any weapons which it works with, as none of your weapons are explosives, grenades (which would probably fall under explosives, no?), or have the Explosive property.
• Heavy Calibre: None of the weapons are two-handed. That's a specific weapon property. The closest you get is the mini cannon being heavy, but that puts small characters at a disadvantage, which is not the same.
• Marksman and All-rounder are probably fine, if a bit generic. But I think the complaint mainly comes from the others being non-function at the moment.

Skill shots

It's hard to give a full evaluation with modular options like this, but it seems fine. It's also hard to evaluate power when the class has so little else, which makes some strong effects here be fine, just so the class has something. Some of these might step a bit on the toes of other classes, such as the Fighter's Second Wind and Monk's Stunning Strike. Might be something to be aware of.

I will note that Skill shots known is absolutely worth a column in your class table, it's written very much like a subclass right now. You might also want to smooth out the progression, so it's closer to one more every 4 levels, rather than 2 more every 8.

Steady Hand

While this might be slightly weird, not moving on your turn for a small bonus to attack is fine. I might have expected this to be the bonus-to-attack Shooting style, as the biggest problem here is that those stack, which is starting to be a problem for bounded accuracy. Having multiple bonuses to attack roll is generally outside the design of 5e.

Of note, being prone gives disadvantage on attack rolls, and having +2 does not enough to mitigate that. And mitigating advantage sounds dangerous, but maybe something you could look at.

Regenerating skill

This feature probably generates way too much resources, though it's hard to tell. I'd want to keep an eye on this during play tests, it would be quite possible for a gunsmith to always have skill points available, which is particularly concerning considering the self healing available through them.

Extra Attack

Then we too late come to extra attack. I'm sure you're aware that other martial classes (that get it) get it at 5th level, and that's with good reason. In short, 5th level is the step up to tier 2. That means Extra Attack, cantrip scaling, and more powerful enemies. And your character is lagging behind that. Getting the 3rd attack at 13th level is probably fine, as there's a lot more flex to tier 3 progression (though generally non-Fighters don't get the third attack, you needn't be beholden to that).

Bullet Time

Very few notes. I'm not sure how strong it is, but it's hard to say as things stand. I will note regarding the 20th level upgrade that reactions generally need some kind of trigger (and arguably need them?) so it's fairly loosely defined, though making 5 attacks (assuming you have the loaded bullets for it) at reaction speed is very powerful.

Other notes

I have a couple of other notes which don't necessarily relate to balance, but more for clarity :

• Gunsmithing schematics known should be its own column in the table (along with skill shots known), it would just be much clearer.
• Speaking of the class table, rather than purely using a numerical improvement on features (eg. "Gunsmithing(2)") you should give a super simple reminder of what the improvement is at that level (eg. "Gunsmithing (+1)), or just say "Improvement" if it's not easily numerized. For the example of Gunsmithing, reading the table I'd assumed you get two uses, or now have two smithed guns, which is very different from what the feature improvement is.
• Laying Gunsmith's tools as Tinker's tools is fairly strange. You could just define something to be gunsmith's tools, or you could have the class use tinker's tools. Right now you technically define the class to get proficiency with a book (and a bottle of gunpowder) which is just kinda weird.

A final suggestion

You seem to have certain specific things you're looking for, but wanting to largely use preexisiting systems (ie. other homebrew) and I absolutely get that, I think it's a good idea. You may be better served by taking that homebrew and adding your own features replacing, or going by the thing you've linked and adding it as an option in its exisiting modularity. Though the applicability of this is gonna depend on which things you're actually looking for.