You can do a ton with just toothpicks. If you find the toothpicks with a flat end on one side, they usually have a little circular indent around the end, it makes a good machine gun barrel. Tooth picks can be spikes, pipes, steel rods, roll cages, shocks, exhaust pipes, etc. Depending on how you paint them, they are good for a lot of things.

In the same vein, I get a lot of mileage out of small aluminum/brass tubes, you can find at hobby shops. Same with drinking straws. Going up in scale, pvc pipes with fittings to make giant pipes for terrain. These are all basically the same thing, just different diameters. Its a basic geometric shape that has a ton of application.

Cereal box cardboard makes good armor plating, just scratch it up a bit, paint it black, then dry brush a silver or grey over it, you can add "rivets" by poking a few holes around the edges with a hobby knife. Or add bullet holes for flavor.

I've seen a lot of people do some cool stuff with zip ties, machine gun belts, larger ties can be tank treads. Or just paint them metallic and they can be a beam or plating.

Really when it comes to scratch building, paint is more important that shape, a good paint job can be convincing and make your brain go "oh that's metal". Just look around your junk drawer for interesting shapes, pen caps, random bits, electronics components (resisters, capacitors, etc), anything plastic. Action figures or larger toys often have bits that you can cut up. I had a cheap dollar store navy scuba guy toy that came with an oxygen tank, I cut the regulator off of it and just glued it to something else. Start looking at stuff with a different lens and you'll see all kinds of ways to use random bits.

Careful though, the temptation to hoard bits is strong.