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.