I concur on a gladius (which is the same size as a Celtic leaf blade, Greek hoplite, Swiss baselard or 18th century artillery short sword) as a good choice in swords. It’s about the length of one joint of the arm, so it becomes an almost perfect extension and usable fairly instinctively. It works better with a shield–1/2 to 3/4 plywood. A basic one can be cut from thin leaf spring stock (1/4″ or 3/16″) or riding mower blades. It works best in formation, but that’s unlikely to be a scenario in the future.
Swordsmithing more than bladesmithing is a very complex task, not for the beginner. Heat treatment is critical, and there’s a lot of metal to move. Grinding one takes longer and will waste some metal (more than half), but shavings can be recycled or melted down. Grinding means less chance for impurities to seep into the metal, and takes only a file or a stone (such as the curb).
Smithing of locks for muzzleloaders isn’t too complex, though it takes some skill tempering, but barrels are a task in themselves. What many re-enactors use for cheap functionality is high-pressure plumbing pipe. Instructions for building a rifling cutter are available in the out of print Foxfire books and others. It’s time consuming but not too complex. Be warned that this pipe will handle blackpowder, but will burst with more modern propellants. With a lathe, transmission shafts or other chrome-moly steel (4140 or similar material) can be bored and turned into good barrels for modern cartridges.
The Chinese repeating crossbow, which I have handled and shot, was intended for use by massed peasants. It suffers from several problems. First, it cannot be aimed well, as the mechanism is above the stock and (second) must be worked while shooting. Third, it is not very powerful, so fourth, it lacks range. Against even thick leather, it is unlikely to penetrate. Fifth, the mechanism is complex. However, an earlier Greek mechanism was built as a ballista for rapid firing bolts. This is a great way to disperse a crowd in a hurry–dropping a dozen spears into the midst will certainly make any charge scatter. And obviously, even a hand-held one has psychological effect for the rate of fire, especially against unarmored people. I would prefer accurate shots at greater range, however, and when the magazine loading time is taken into account for the repeating crossbow, a good recurve in practiced hands is more effective and simpler. (For note, I have recurves, longbows and crossbows in the house and compete at re-enactments at an adequate if not impressive level. I am generally biased toward recurves for rate of fire, but I prefer the crossbow if I have time to make the shots count.)
I just saw this video on archery (it’s in Korean). The interesting part for me is the great slow-motion shots of arrows in flight, showing the oscillations that they must go through in order to fly “Straight.” The arrow is propelled straight by the string, but must bend around the limb of the bow. This is something that arrows must be designed for for best accuracy. – Michael Z. Williamson, (in sword maker rather than sci-fi writer mode)