That is very noble of you to pick that project for your science fair project. It is good to try to be a good person, in addition to being good at science and math.
I'll offer a suggestion for the sensor, but then I need to offer some important practical advice about this.
To detect metallic weapons like pistols and rifles, you can use a low-power circularly polarized radar that chirps through wavelengths that correspond to the physical sizes of such weapons. Your radar receiver would pick up radar returns from such metallic weapons. You would probably use an antenna array to steer your sensor and the receive antenna beam over an area in front of the drone. This is well-developed technology.
But beyond that, since metallic weapons are fairly easy to detect, terrorists will do their best to assemble weapons that do not involve much metal. Pistols and rifles can be made with ceramic materials, or just 3-D printed using strong plastic materials (dosposable weapons). IEDs can be made with ceramic or plastic or other casing materials, and chemical and biological weapons do not need metallic casings.
But the problem with doing research about these other weapon modalities to try to design sensors for them, is that if you spend much time doing that on the Internet, you will start raising red flags with the monitoring services that watch for terrorist activity. I would recommend that you not do much research into detailed terrorist weapon detection, because that could put you on some watch lists that I would never want to be put on.
Just some friendly advice. You can certainly get a lot of mileage from a standard beam-steered, chirped radar antenna array with signal processing to detect metallic pistol and rifle weapons with your drone for your project, IMO. The biggest problem is that such an antenna array will need to be about the size of the weapons you want to detect, so your antenna array will be pretty large compared to size of your drone itself.
Have fun, and stay smart!