Can Hunter's Mark bonus be applied to the Eldritch Cannon's attack?

Title pretty much says it all. On my turn if I use Hunter's Mark to mark a target, I add an extra 1d6 to my attack rolls against that target. Then on my next turn when my cannon takes a shot on my BA command, does it also add 1d6 since the cannon is still "my" attack?

That means at level 5, an artillerist could (by the second turn) stack hunter's mark, true strike, arcane firearm, regular weapon damage, cannon damage, and hunter's mark a second time!?

The description of the Eldritch Cannon's Force Ballista thing says "make a ranged spell attack originating from the cannon", so it sounds to me like technically it's the Artificer making the attack, not the cannon (which isn't a creature and thus can't really make attacks on its own). In which case, yes, Hunter's Mark would apply if the attack hits.

Hunter's Mark would certainly also apply to a True Strike attack. Whether Arcane Firearm would also apply is a bit of a gray area; it says it works "when you cast an Artificer spell through the firearm", which I think is intended to mean when you use it as a spellcasting focus. You're not technically doing that when you cast True Strike — the weapon is the material component in that case, not a focus — so some DMs might not allow that. I probably would unless it looks like it's going to deal a truly ridiculous amount of damage.

The description of Arcane Firearm says that whichever weapon you carve your sigil onto can then be used as your spellcasting focus:

"You can use your Arcane Firearm as a Spellcasting Focus for your Artificer spells."

Quote from MongooseGef >>

The description of Arcane Firearm says that whichever weapon you carve your sigil onto can then be used as your spellcasting focus:

"You can use your Arcane Firearm as a Spellcasting Focus for your Artificer spells."

Yes, that's true. The point is that because True Strike has a costly material component, you can't use a spellcasting focus with it at all. As it happens, the material component is the same physical object that you could use as a spellcasting focus for other spells, but you're not using it as a focus in this case, so one could argue that Arcane Firearm doesn't apply.

Again, I don't think it's actually worth quibbling about in practice, but some DMs are sticklers for that kind of thing.

Reading the spells, I think yes, Hunter's mark could apply both to your firearm attack and your force ballista cannon attack.

That said: you'd have to spend a bonus action to cast it and a bonus action any time you needed to move it. So on those turns, you'd be giving up your cannon attack. I suspect in practice it will rarely be worth it, unless you're fighting a lot of rounds against a big boss. The cannon needs your bonus action every turn.

I don't see an issue with True Strike and Arcane Firearm damage stacking. True Strike is an artificer spell and it's being cast through your Arcane Firearm. Note that from the wording Arcane Firearm damage die doesn't double on a crit.

Quote from FoolishMortal18588 >>

Reading the spells, I think yes, Hunter's mark could apply both to your firearm attack and your force ballista cannon attack.

That said: you'd have to spend a bonus action to cast it and a bonus action any time you needed to move it. So on those turns, you'd be giving up your cannon attack. I suspect in practice it will rarely be worth it, unless you're fighting a lot of rounds against a big boss. The cannon needs your bonus action every turn.

I don't see an issue with True Strike and Arcane Firearm damage stacking. True Strike is an artificer spell and it's being cast through your Arcane Firearm. Note that from the wording Arcane Firearm damage die doesn't double on a crit.

 That's a good point. In that case it would be better to cast Hex, as it follows the target automatically (freeing up that precious BA) and still does 1d6 additional damage. Plus, they have disadvantage on an ability roll of your choice. Too bad it can't be applied to saves!

Quote from MongooseGef >>

Quote from FoolishMortal18588 >>

Reading the spells, I think yes, Hunter's mark could apply both to your firearm attack and your force ballista cannon attack.

That said: you'd have to spend a bonus action to cast it and a bonus action any time you needed to move it. So on those turns, you'd be giving up your cannon attack. I suspect in practice it will rarely be worth it, unless you're fighting a lot of rounds against a big boss. The cannon needs your bonus action every turn.

I don't see an issue with True Strike and Arcane Firearm damage stacking. True Strike is an artificer spell and it's being cast through your Arcane Firearm. Note that from the wording Arcane Firearm damage die doesn't double on a crit.

 That's a good point. In that case it would be better to cast Hex, as it follows the target automatically (freeing up that precious BA) and still does 1d6 additional damage. 

Hex and Hunter's Mark work the same in this regard.

Oh, yes, I suppose they do! Presumably, this combo would only be used on a big bad that requires multiple rounds to defeat and not on lower level grunts.

I guess each spell has its advantages.

Quote from wagnarokkr >>

Quote from MongooseGef >>

The description of Arcane Firearm says that whichever weapon you carve your sigil onto can then be used as your spellcasting focus:

"You can use your Arcane Firearm as a Spellcasting Focus for your Artificer spells."

Yes, that's true. The point is that because True Strike has a costly material component, you can't use a spellcasting focus with it at all. As it happens, the material component is the same physical object that you could use as a spellcasting focus for other spells, but you're not using it as a focus in this case, so one could argue that Arcane Firearm doesn't apply.

Again, I don't think it's actually worth quibbling about in practice, but some DMs are sticklers for that kind of thing.

So for ordinary spellcasting you're right True Strike can't be cast with a spellcasting focus as it's material component can't be replaced due to having a specified gold cost.

But the Artificer's Spellcasting has the feature Tools Required which states the following:

You produce your Artificer spells through tools. You can use Thieves' Tools, Tinker's Tools, or another kind of Artisan's Tools with which you have proficiency as a Spellcasting Focus, and you must have one of those focuses in hand when you cast an Artificer spell (meaning the spell has an M component when you cast it).

This specific wording makes casting Artificer spells (with Artificer spellcasting) without a focus an impossibility for the Artificer. They MUST use a spellcasting focus. 
If True Strike is impossible to cast without a focus and Artificers can't cast any Artificer spells without a spellcasting focus then Artificers can't cast True Strike at all.

But True Strike is an Artificer spell, it's on the Artificer's Spell List. So clearly the intent is for it to be castable by an Artificer.

But how? It turns out rather simply. The feature adds an M component when you (the Artificer) cast the spell.  You then replace that added component with a Spellcasting Focus, not the weapon worth 1+ cp which cannot be replaced.

The same way that an Artificer can cast Acid Splash with a spellcasting focus despite the spell not having any material components at all.

When a Wizard casts Acid Splash it's components are V, S.
When an Artificer casts Acid Splash it's components are V, S, M (the added component*).
* must be replaced with a spellcasting focus.

When a Wizard casts True Strike it's components are S, M (a weapon with which you have proficiency and that is worth 1+ CP)
When an Artificer casts True Strike it's components are S, M (a weapon with which you have proficiency and that is worth 1+ CP and the added component*)
* must be replaced with a spellcasting focus.

So an Artificer can absolutely cast True Strike, they just need both a weapon AND a focus. If they use a replicated magic weapon, that weapon can serve double duty as a replicated magic weapon can be used as a focus. If an Artillerist's Arcane Firearm is also a weapon (such as a Staff, or Martial Ranged Weapon) then their firearm can serve double duty, and I'd argue the Arcane Firearm bonus would also apply.

 Edit: DnDBeyond, what do you mean the difference between a legacy spell tooltip and a 2024 spell tooltip is whether or not the tag is plural? This is obtuse as hell.

Fair enough, though I stand by my opinion that none of this is worth actually caring about in practice.

Quote from wagnarokkr >>

Fair enough, though I stand by my opinion that none of this is worth actually caring about in practice.

 I mean in practice its a difference between, will I or won't I use the spell at a specific table.

It's not something I enjoy arguing about, but its come up enough on these forums that I've managed collate a well rehearsed response.