i just can't wrap my head around it, is Arcane Firearm an automatic feature or a choose feature.

You know how to turn a wand, staff, or rod into an arcane firearm, a conduit for your destructive spells. When you finish a long rest, you can use woodcarver’s tools to carve special sigils into a wand, staff, or rod and thereby turn it into your arcane firearm. The sigils disappear from the object if you later carve them on a different item. The sigils otherwise last indefinitely.

You can use your arcane firearm as a spellcasting focus for your artificer spells. When you cast an artificer spell through the firearm, roll a d8, and you gain a bonus to one of the spell’s damage rolls equal to the number rolled.

but it doesn't say if you choose the arcane focus or not.

What? I'm not sure if I understand what you're asking, but if I do, then yes, you can choose the arcane focus you use the feature on. Why would you believe otherwise?

Yes, you can choose which focus to use the feature on.

You get to specifically choose a "wand, staff, or rod" .. you can't choose any other kind of arcane focus like a crystal or orb.

This one specific item becomes an "arcane firearm" and can be used as an arcane focus for your artificer spells (normally artificers can use only a set of tools or an infused item as an arcane focus)

Whenever you use THAT specific item (the arcane firearm) to cast an artificer spell then you get to add a d8 to one of the spell's damage rolls.

You can choose to use a different focus without that bonus to cast your spell if you like but I'm not sure why you would choose that.

Keep in mind that unlike other casters, EVERY artificer spell much be cast with a set of tools or an infused item as a spellcasting focus. Artificers can't use the other options. The Arcane Firearm feature gives the Artificer another type of item that they can use as an arcane focus for their spells. 

"Tools Required

You produce your artificer spell effects through your tools. You must have a spellcasting focus—specifically thieves’ tools or some kind of artisan’s tool—in hand when you cast any spell with this Spellcasting feature (meaning the spell has an ‘M’ component when you cast it). You must be proficient with the tool to use it in this way. See chapter 5, “Equipment,” in the Player’s Handbook for descriptions of these tools.

After you gain the Infuse Item feature at 2nd level, you can also use any item bearing one of your infusions as a spellcasting focus.

Quote from up2ng>>

It doesn't matter what those items are.  First, obviously not all wands, staves and rods are arcane foci.

Well, that's the only in-game definition we have for those items.

But more importantly, those items have been turned into something else with this feature -- an item called an "arcane firearm".  An arcane firearm is not an arcane focus.  It is a tool.

What? Being "turned into" something doesn't automatically remove all other facts about an item.

Here's an example of that not being how things work. Pact of the Blade says that you can "transform a magic weapon into your pact weapon." Assuming you don't arbitrarily decide "turn into" and "transform" have different meanings, your claim means that the magic weapon would suddenly cease to be itself because it has been transformed into a pact weapon. That's wild and obviously untrue.

 

An artificer might have access to all sorts of items and objects on his person that can be used as a spellcasting focus -- each of these might grant some sort of different and unique bonus to the casting of a spell and he can pick and choose which one to use as the situation dictates.

Where's this great list of unique items that all give different bonuses to an Artificer's spellcasting? I certainly haven't seen it.

I believe the relevant point is that Artificers cannot use arcane foci as a spellcasting focus, with the Arcane Firearm being a specific exception. Regardless of whether or not you consider the object to have been an arcane focus prior to being augmented by the feature, an Artillerist may only treat it as a focus for Artificer spells after the augmentation.

As far as the artificer feature is concerned, the item affected by the arcane firearm feature, a rod, staff or wand are arcane foci and can be used as such by a class that uses arcane foci like a wizard. An artificer does NOT use an arcane focus however. An artificer can not pick up an arcane focus and use it to cast their spells.

On the other hand, a rod, staff or wand that has been made into an arcane firearm CAN be used as a spellcasting focus for casting the Artillerist Artificer spells of the character who created it. (It could also be used by a wizard to cast their spells since the object is still also an arcane focus but the wizard doesn't get any benefit from the arcane firearm feature).

The bottom line is that artificers can NOT use an arcane focus to cast their spells. They can use a set of tools with which they are proficient, an item they have infused, or in the case of the Artillerist artificer their Arcane Firearm (which is a spellcasting focus for the artificer but not an arcane focus FOR the artificer though the object could still be an arcane focus for someone else since the object used to create the Arcane Firearm was also an arcane focus and it doesn't lose that property).

P.S. Since I was part of the problem calling the artificer spellcasting focus an "arcane focus", I edited my post above to replace the word arcane with spellcasting. Arcane focus is the name given to the spellcasting focus wizards use to cast wizard spells.

but it's not like the pact weapon for pact of the blade feature, is it?

Quote from xtgc6tkfxj >>

but it's not like the pact weapon for pact of the blade feature, is it?

 
What do you mean?

as in you can choose what item it is

Quote from xtgc6tkfxj >>

as in you can choose what item it is

 
Yes. I already said that on page 1. The player gets to choose which item their PC transforms into their Arcane Firearm. They have to manually carve runes onto it, that means they have to actually choose which one they pick up to carve.