Those real world weapons are a different kettle of fish. (I can not confirm for security reasons if I have been into related facilites).while my science skills may be low... I would like to point out there already exist a few IRL short range laser anti-drone weapons (royal navy Red Sea & ukraine around a few electricity centrals, US & French Rafael lazers) wich may explain some noise er... situations. I quickly googled those weapon systems and they're silent, very focussed and er... only visible when burning the target.
For starters they and other weapons of similar types are sustained directed energy beams which is not a practial use of a typical military long arm. It is too cumbersome to use in agile infantry combat. Targets move too fast at too close a range to lock on for long enough. Those weapons rely on sustained contact, your ganger will be able to move out of the beams area of effect before can kill them. Those systems also require dedicated rather bulky computers to function technically and importantly maintain the target lock. The Imperium uses rotting skulls for computers.
For a Handheld rifle type weapon to be effective on a human it would require a much higher intensity light beam at a hotter burn for a much shorter duration.
Todays laser weapons are for attacking Drones Heilcopters and other arial targets a lot of the damage is to the electronic systems and internal mechanisms. Humans have much more robust electromagnetic systems than todays inorganic electronics. EMPs and electronic damage does little to humans.
A real life laser would do nothing apart from heat up a solid shot cannon ball. Various personal lasers in development are at best a non lethal crowd control weapon with some, banned, blinding properties. To bring it up to lethal, arm the Imperial guard, levels it would require massive increases in power for the gun that just is not similar to the dazzlers and electro lasers today, its mice to elephants.
The larger ship and turret weapons in use and development today are arial defence weapons and the weapons themselves are not silent. Just listen to your PC or laptop. They have a tiny fraction of the juce going through then compared to a one of those. Each weapon will have a distinctive noise. It's not a crack or a bang of a proectile but it is still there. To massively ramp up the output to an anti human weapon it would increase the noise level.
Not a practical use on humans with 21st century personal weaponsmicrowave based systems are also used against slow moving drones (basically those systems 'laser burn' or 'focus microwave' slow moving targets)
As I discribed in overview.This may be separate from lower intensity 'targeting' lazers and maybe 40k's laser differ in other points (stronger, more light in a fraction of a second, sudden overheating lens creating air 'pop' or a directional 'fast blast, part of the lens being functional and part creating 'effect lights')
Once you increase the power and capability of a process it does not just track that the previous conditions would be the same at a lower level or even that they increase at a Linear rate. The maths just does not move in a Linear line it is a ramping up curve.
Apart from some theatrical licence over the exact colours (The Bright green laser bolts look better on film than pale) and some slowing down for dramatic effect Star wars actually got its "lasers" right in basic terms. The general laser physics has not really changed since the 1960s just improved engineering

