Learn French: Ils auront lancé chaque flèche sur la bannière. - They must have launched every arrow at the banner.

This is the epistemic use of the future perfect, often called the futur de probabilité. Instead of talking about the actual future, it is used to express a strong assumption or high probability about an event that already happened in the past.

Could this exact same French sentence mean 'They will have launched every arrow' in a different context?

Yes. Without context, Ils auront lancé could be the standard future perfect describing an action completed before a future point in time. However, to mean 'will have,' you usually see a future time marker in the sentence, like avant l'aube (before dawn).

Why use sur la bannière rather than à la bannière for 'at the banner'?

When verbs involving throwing, shooting, or launching (like lancer, tirer, or jeter) are directed at a physical target, French typically uses the preposition sur. Using à would sound more like giving or handing something to the banner.

What is the nuance between using chaque flèche instead of toutes les flèches here?

Chaque flèche translates closely to 'each arrow' or 'every arrow,' emphasizing them individually. If you used toutes les flèches (all the arrows), it would describe the arrows as a collective group. Both are correct, but chaque highlights the individual shots.