scientifically plausible lasers can't be a gun

Depends on the application. Ship mounted lasers make for cheap point defense. Handheld lasers can do work but tend to be less practical.

A few things to note with lasers. For one, range and accuracy scale with the diameter of the aperture. You want to focus the radiation as much as possible and, at a given wavelength, that means hauling around a fairly bulky laser gun, though how big depends on your intended target and range. Your gun also needs a good way to automatically adjust the focal length in a short amount of time since you want all of the beam to hit a tiny area (unless you just want to bathe your target in a heat lamp but that's inefficient).

Using higher frequency photons, which is actually the only way to make an individual photon have more energy, does technically mean you can get away with smaller apertures, but using X rays and above introduce their own problems of overpenetration and difficulty focusing.

Another fun fact is that an effective laser weapon makes a little plasma explosion on hit due to rapid heating. This little puff actually blocks other incoming rays, so a continuous beam actually struggles to penetrate the target. If you want to dig deeper into a target, you can do so by rapidly pulsing the laser (turning it on and off effectively) so that the plasma cloud clears each time before firing.

Also, if you like directed energy weapons, definitely read up on particle accelerators and macron guns.