I'm currently working on the next campaign that I will be DMing for a group of my friends but the era it is set in is somewhat steampunk-ish. My goal is to introduce guns and such but I've been wanting to make a warlock-like subclass like the hexblade or make pact weapon that features the gun and uses it as the spellcasting focus. Maybe this is easier than what I think it is, but I was wondering what some of the other DMs or players might think of how to do this? I thought the Arcane Archer for the ranger class would be a simple enough change...just exchange the bows and arrows for pistols and such. Do ya'll have any ideas or thoughts?

I think Arcane Archer is a Fighter sub-class rather than Ranger; under Fighter you also have the Critical Role Gunslinger which is a straight up firearm using class, though it's a little weird mechanically (custom weapons, a Misfire mechanic that while flavourful can be a bit of a pain).

I've been working on my own alternative (see my signature) to make it use regular DMG guns rather than custom ones, tweak how some of it works, and give a sample crafting system for custom weapons (not specific to the Gunfighter, any class with Firearms proficiency and Tinker's Tools could use it, or other firearms users could get guns made by the party's Artificer or Gunfighter etc.), I've not published it yet, as it's a big homebrew with lots of parts so I'm giving it the "proof reader it a thousand times" treatment till I'm sure it's free of bugs.

As for tweaking Warlock to use a firearm, using it as a focus I don't think would be much of a problem, but I'm not sure how balanced pact blade abilities would be with it (since they're specifically for blades), but the other pacts should still go well enough with a shooty Warlock (Flintlock? 😄), or you can still do Pact of the Blade and take a pistol and then have them both drawn at once if you're switching from slicing to shooting.

If a Warlock takes the Pact of the Blade, they can choose the Improved Pact Weapon Eldritch Invocation. You can simply make the same exceptions for bows and guns that you were already planning for Arcane Archer (which is actually a Fighter Subclass, not Ranger).

Improved Pact Weapon

 Prerequisite: Pact of the Blade feature

You can use any weapon you summon with your Pact of the Blade feature as a spellcasting focus for your warlock spells.

In addition, the weapon gains a +1 bonus to its attack and damage rolls, unless it is a magic weapon that already has a bonus to those rolls.

Finally, the weapon you conjure can be a shortbow, longbow, light crossbow, or heavy crossbow.

See thats what I was originally thinking about the sub the pact of the blade for the gun but I wanted different ideas or takes on it. Sorry, i meant the fighter, not the ranger!

Well, I wouldn’t personally let the regular Pact of the Blade count for guns, only the Improved Pact Weapon since it specially calls out bows and crossbows. If you’re replacing bows with guns for the Arcane Archer, it kinda makes sense.

You might get some inspiration from the Artificer Class. There might be something there that could inspire you to do something different for Warlocks.