"I do not feel particularly beholden to the quality of their catch and there are only so many places where there's river access."

That's not very prosocial of her!

...anyway, they can have the rest of this conversation while they walk to the river.

Or fly, as the case may be.

Yes, yes, she's very jealous. They cut through some suburbs and go down some stairs and there's a nice big river, and some fishermen.

If she can get a sufficiently flat and rigid platform, Tanya can fly her alongside! This is faster than walking and therefore efficient. (It's not actually efficient in terms of mana usage, but as long as Tanya doesn't have to do anything else at the same time and they're not actually going fast it's fine.) It might also looks impressive, although Tanya is no judge of local customs.

Tanya tries picking up a rock and shoving it with the flight spell. It tumbles as it flies and lands... not really where she wanted it to.

The flight spell can't adjust to the rock's irregular shape, let alone its internal variations of density. If Tanya could determine its exact center of mass, and ideally if it was also shaped in a way that wouldn't make it tumble and deviate as it flies, then she could aim it. The computation orb can't do its namesake job here, because its flight spell really wasn't built to target random rocks. The latter part doesn't matter for a sufficiently massive rock, but carrying massive rocks around is impractical.

Tanya can find a few especially well-shaped rocks and figure out an approximate solution for launching each one by trial and error. This is the antithesis of standardized ammunition and she does not want to rely on it.

She tries giving a stone flat sides with a cutting laser. This takes annoyingly long because the stones fracture from the heat if she tries to go any faster but the result is more predictable in flight. If she wants to approximate round stone balls she'd probably have to work for an hour on each. And then she'd have to carry them around and recover them from enemy bodies and clean them for reuse. Even muskets weren't this annoying to use!

How hard can she fling a kilogram projectile, though? The flight spell only works up to a meter and a bit away from her body. Heavier projectiles take longer to accelerate and so gain more momentum, but if they're too heavy they won't go far enough. Tanya tries to figure out what mass is needed for an effective range of fifty meters... and quickly runs into another limitation: the flight spell refuses to apply more than 10g of acceleration to any object, regardless of its mass. (This is a safety feature!)

With an effectively constant acceleration (up to a projectile mass of a hundred kilograms), she can't make a railgun: light bullets won't go any farther than heavy ones. She can, however, make a 100kg bombard at a forty-five degree angle with a radius of twenty-three meters, if only she had a hundred-kilogram rock on hand.

All in all, it doesn't seem terribly useful unless they need to siege a miniature castle. She reports this to Belmarniss.

"We could get you sling bullets, would those be better?"

"No, because they won't fly any faster than heavy rocks, which means they'll still have a range of twenty meters. And that's in a ballistic arc; a flat trajectory wouldn't make it past ten meters. You'd be better off using an actual sling."

"Gotcha. Okay. Fire elemental now?"

"Alright. Your spell has a limited duration, right? Can you tell if the construct is damaged or how much or do we only learn something worked if it 'dies'?"

"If it gets injured it'll be visible, but I don't get direct feedback on it. It'll only last about thirty seconds."

"Then I'll be fast. I can replay the record from my orb later if you miss something."

"Sounds good. I'll put it over there?" Point.

Tanya positions herself so her shots will go through the indicated location and into the river at a shallow angle. "Go ahead." And she speeds up her thinking and reactions; she can tolerate a threefold speedup for thirty seconds.

"Summon Monster."

Here is a snek made of fire. Hiss.

Heat: no apparent effect. Visible light: no effect, up to levels where ambient reflections would start being a problem for Belmarniss. (The snake isn't very reflective, so those are quite high levels.) So far as advertised.

What does an ordinary submachine gun bullet to the head do?

That kills the snek. It vanishes.

"Nice."

"One standard shot, not explosive or anything. If it was like a regular snake and I destroyed the head then that makes sense, but snakes aren't generally known for being armored. Other creatures might require an armor-penetrating or explosive spell on the bullet, but this wasn't harder than killing an ordinary snake. Except for how light didn't work on it and it could presumably burn me if it got near, ordinary snakes would be stopped by my barrier."

After all the buildup this was rather a disappointment. Not that Tanya is disappointed that her enemies local wildlife is easy to kill, of course, but this doesn't fill like it demonstrated much to Belmarniss -

...Tanya suddenly realizes she is being deeply silly. Belmarniss is a civilian! She happens to be a mage and to know some spells that can be used to kill people, but that doesn't give her the mindset of a soldier. She's a young girl who wants to leave home and traverse unexplored wilderness to reach the mythic land of sunshine and libraries, and she's afraid of fire snakes (and possibly regular ones) on the way. That is an extremely legitimate and understandable fear! Snakes really are dangerous to people! Even to mages, if they can't fly or shield themselves or if they happen to be asleep! Of course Belmarniss wants reassurance that the literally alien foreigner escort is competent!

Now she's feeling embarrassed. What was she thinking of, showing off to a civilian? 

"...did you want to summon another, different target?"

"Sure, do you want another kind of elemental so you can see how your light thing stacks up against stuff that's not immune?"

"All right. Do you think an air elemental might be immune because of being transparent? Is it even visible?"

"They're visible. Not as obvious as a fire one but you can see 'em. I don't expect them to be immune."

"Understood. Please go ahead."