Alternatively, a Hamon laser pistol, which could be accomplished fairly easily.
First, you get a Red Stone (there were others besides the 'Super Aja', after all, even if they weren't as potent) to serve as the
gain medium[1], which you then place inside a tube with some lenses (and maybe certain gases; this is where you make Speedwagon R&D's decade by giving them clearance to research magical sunlight laser guns, assuming they don't have any projects like that already) to focus the beam of energy from the Red Stone.
Then, you add a pistol grip with a metal cylinder threaded into it at a perpendicular angle, with a wire wrapped around its midsection (inside the grip itself) which then extends up into the tube and touches the back of the Red Stone. This creates a channel that Hamon Users can use to feed energy into it through the hand holding the weapon, thus making the wielder do double duty as the
pump source[2].
Now, it's not entirely clear whether the resultant beam of energy would actually conduct Hamon into its target, but it would at least be something worth investigating - especially considering that the bodily component that reacts most readily to the heat of a laser is blood, along with other bodily fluids, which we already have proof are quite capable of conducting Hamon.
And even if it just produces a high-intensity laser, then you can just stick a high-powered flashlight at the back of the barrel and now you have a laser rifle that can be run off of a lithium camera battery. That's a hell of a consolation prize.
[1] The bit of the laser pistol that the energy source applies its energy to in order to make lasers happen.
[2] The bit of the laser that provides energy to the gain medium, so that lasers can happen.
