While writing an answer for this question I came across a problem with how the Eldritch Cannon and the Fortified Position features interract.
Eldritch Cannon states:
Once you create a cannon, you can't do so again until you finish a long rest or until you expend a spell slot of 1st level or higher.
Fortified Position states:
You can now have two cannons at the same time. You can create two with the same action (but not the same spell slot), and you can activate both of them with the same bonus action.
I use an action to create two cannons:
- Cannon#1 is not a problem. It's either the first cannon I'm creating since I took a long rest, or I expended a 1st level spell slot some time prior to the current action.
- Cannon#2 is a problem. I have not taken a long rest, nor have I expended a 1st level spell slot since creating cannon#1.
The Eldritch Cannon feature doesn't provide me with a way to expend a spellslot, but it prevents me from creating a second cannon before I expend a spellslot.
The Artificer has no means to expend a spell slot other than to cast spells. So I need to somehow cast a spell in the middle of my action.
My understanding is that I cannot generally break up my action with a bonus action, so I cannot expend a spell slot by casting a bonus action spell between the creation of two cannons.
A reaction can interrupt an action so it could occur between the creation of two cannons, but getting the opportunity to cast a reaction spell at the right time seems excessivily unlikely and difficult.
Therefore my question is:
When an artificer uses its action to create two Cannons, how can it easily expend a spell slot between the creation of the first and second cannon?