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One of my players is planning on playing Matt Mercer's Gunslinger fighter archetype.
If their character gets proficiency in tinker's tools before 3rd level (such as from the Guild Artisan background, PHB p. 132-133), can they start crafting guns - but not have proficiency in using them - before 3rd level?
The description of the Gunslinger's Gunsmith feature states:
Upon choosing this archetype at 3rd level, you gain proficiency with Tinker’s Tools. You may use them to craft ammunition at half the cost, repair damaged firearms, or even draft and create new ones (DM’s discretion). Some extremely experimental and intricate firearms are only available through crafting.
The ability to draft and create firearms using tinker's tools is up to DM discretion, including whether it's even possible and how one would go about doing so. This is true even after the feature gives you the ability to craft ammunition and repair damaged firearms at 3rd level. As such, any such use of tinker's tools before 3rd level would be up to DM discretion, albeit unlikely to work reliably.
Personally, as a DM, I'd probably rule that you don't have a good enough understanding of firearms before you choose the subclass to craft and create weapons using tinker's tools.
There are a lot of misconceptions related to Tinker's Tools, and they were actually relevant to a character I just started. Tinker's Tools are actually a lot more mundane and useless than most players believe.
Many players believe that Tinker's Tools allow you to create simple machines or work on grand devices on a project, but the actual use of Tinker's Tools is to be a medieval, mundane handyman. A few things that Xanathar's Guide to Everything mentions include doing shoddy repairs on a sword, fixing the leg of a table, or patching a hole in a cloak.
Even with a 20 DC, the best you would be able to do would be to temporarily create an improvised item/upgrade (like an improvised shield out of wood).
The "adventurer" benefits of Tinker's Tools come from outside sources, such as a Gnome's racial trait, the quicksmith feat in the Kaladesh article (not exactly Tinker's Tools, but almost identical), or the Gunsmith bonuses from the Gunslinger archetype.
These are not the default, and without those sources, a player should technically not be able to utilize Tinker's Tools to create, but only to repair. And those things you repair should be things common to the lowest fantasy settings possible.
I asked mine, and he allowed me to replace my background with one that duplicates the Gnomish Tinker's Tools traits, along with several other Gnomish-esc things. At the time, he was under the impression that the advanced abilities of Tinker's Tools were inherent, until I basically gave him this same answer.
