okay i've looked on some of the Black Library forums and here's what I came up with:
Lasguns laser type isn’t detailed, though it is incredibly powerful.
Laser beams don’t show up in normal conditions, yet a Lasguns blast does so that suggest that the laser is so strong it is super heating the air it is travelling through, and turning it into plasma – though Lasgun lack a thunder clap from the collapsing vacuum column so they may be a duel feed weapons with a partial projection component.
The effect of such a powerful laser strike would instantly vaporise the target area creating a powerful concussion wave from the rapidly expanding gas (a similar technique is use in real science to initiate fusion using high powered lasers to compress tritium pellets).
The concussion wave would be powerful enough to shred bio-matter and internal organs creating a wound that most wouldn’t consider a laser inflicting, but although a low power laser would cauterise a Lasgun would leave huge smoking holes of pulped and ragged flesh.
The other effect is that unlike a gun (ignoring movies completely for the mo), a target hit would be thrown back by the instantly vaporised and rapidly expanding gas sphere – it is literally an explosion.
Ceramite armours (like what the IG have) would be highly effective at stopping such a weapon, but a strike to a limb would most likely be fatal. If the laser struck such an armour the heat would no transfer to the target and ceramite is an excellent insulator, though the energy has to go somewhere so the area in front of the trooper would heat up rapidly, so the air would still push a bit but not like a direct hit.
Philip Sibbering
Phil touches on a point about lasguns that keeps coming up - they seem to combine laser beams with something that has mass. They don't behave like pure energy weapons, since pure energy doesn't have any mass and if the round being fired has no mass there would be no recoil on firing, but there is. Recoil on firing is recorded numerous times in the fiction.
I like the idea that some of the energy of the round is converted to mass when the trigger is pulled, perhaps as the round travels down the barrel, gaining mass as it goes. That way longer barrels produce rounds with greater mass and thus greater stopping power - just like a long-las does.
This mass-element would also cause the ricochet effect of lasrounds seen in the fiction.
The mass-element could be converted back into energy on impact, or perhaps simply vapourised in the explosion Phil describes.
Sholto
We have to remember that the authors are all creating their own version of the 40K universe And a lot of times they conflict with each other. DoW IS Games-Workshop approved (i believe) and so are the books in the BL which describe lasguns WITH and without recoil.
HiPpIe_X
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