I got bored this morning, and so I did some quick calculations about one of my favorite scifi weapons. The Warhammer 40,000 lasgun.

For the sake of ease, I am going to be doing all of this with the Kantrael M36 in mind. This is a pretty standard rifle across the Imperium, and is a good starting point. All lasguns seem to use the same power pack, but with different powers, and so I'm only doing this for a stock variant on normal settings. The power packs hold 19 megathules of energy. I have no idea what a thule is, and I can't find any other mention of it. For the sake of being able to do this at all, I am going to say that a thule is equivalent the the real world Joule. This powerpack, with its 19mJ, is capable of making 150 shots, so I'll divide it. But first, no directed energy weapon is 100% efficient, there is always some heat loss. 95% efficiency is a number I found that makes the effects comparable to the ones shown in lore. This gives us a thermal energy of 120kJ on target, and a heat loss of 6.5kJ. To calculate the damage, I have made a few assumptions. The beam does not lose any power over distance, and the beam is 1 square centimeter in diameter.

A human body has a specific heat of 3.6kJ/kg. The point of impact would therefore be heated 3.3 million degrees Celsius, though it would disappate across the body. This sudden, extreme heat would instantly boil the water content of the target explosively, and would blow limbs off or leave smoking craters in the flesh, as seen in the Ciaphas Cain novels. The wounds likely wouldn't cauterize immediately, and so you would still bleed out even if the massive thermal shock didn't instantly kill you. A lasgun seems to be made of plasic and steel, and so I give it a specific heat of 1.25kJ/kg. With the 6.5kJ of heat loss, each shot would heat the rifle an astounding... 2 degrees Celsius, so the sustained fire often seen is also possible, even without active cooling.