So I see so many people either leave their borrower OCs unarmed, or give them toothpick spears and needle swords, but there's so much more out there.
Firstly, bows. A bow and arrow helped us humans advance as a species, and all it requires is a very basic knowledge of tension physics, which wouldn't be hard to experience in a more complicated society. A little bit more technology, some gearing and more structurally sound cordage and you can make yourself a tiny crossbow, which is easier to train someone to use effectively, and has more stopping power (Especially against armour that an arthropod or reptile would possess). Hell, with elastic bands being everywhere, it wouldn't be hard for a society to figure this one out.
But why stop at tension-powered ranged options? While hard to obtain, there are situations where using chemical-powered weaponry would be viable; gunpowder.
Various forms of gunpowder could be scavenged from human bullets, sure. But fireworks and sparklers could work in a pinch. All a gun really boils down to is a sturdy tube with some boom powder at the bottom and a projectile in front of that. A little bit of tiny blacksmithing could enable the manufacture of gun barrels, and by extension, cannons and small arms. Granted, without more complicated tinkering, I don't think a society would advance to more modern auto-loading guns, but it would be cool.
Now, consider; GIANTS. Even a minigiant in the 10-15 foot range would be a monster with what they could wield. Imagine the power of a warhammer wielded by someone x2 as tall as a regular soldier, or the range granted by a giant-sized spear. A crossbow would effectively be a small ballista, and have you seen what a human-sized greatbow can do?
But as a connoisseur of modern weaponry, I must add: Militaries have adopted larger weapon platforms that would be perfect for a minigiant to manhandle. With the advent of lighter machine guns during and after WW1, and the adoption of miniguns in the 1960s, it would be possible to field them for infantry use, as opposed to our world which requires these to be mounted on tripods or on vehicles.
The MK19 is a 40mm automatic grenade launcher. Imagine giving THAT to your minigiant OC and seeing them wreck havoc.
Please, I'm carrying the giant heavy gunner giant archetype on my back with Eights, and they wear an exoskeleton to do it.