Very impractical for many reasons already mentioned here. I mean air rifles designs are very old (Napoleon famously gave order to execute any enemy soldier foun

Very impractical for many reasons already mentioned here. I mean air rifles designs are very old (Napoleon famously gave order to execute any enemy soldier found with one), and they haven't caught up in any modern military. It turns out its very practical to have fully contained ammo with casing, semi-waterproof, shockproof, can last decades easily and super cheap to make due to economies of scale. Heat dissipation from barrel friction is an unsolved problem, you can't just skip around physical constraints. Also, having variable power isn't very practical - you want a steady point of hit for consistent fire, otherwise you would have to re-adjust gun sights/scope every single time. One of the biggest pain points of even modern air rifles - as the air pressure goes down, so does velocity of the pellets and your precision. I think we will see sooner some sort of plasma guns than standard ammunition shot via similar ways.