I don't think anyone's asked this. I know they made an aluminium 18lb model, but looking at the 34lb steel version; I used to think that the gas out the back formed a high-pressure area which created a kind of virtual breech wall so that some of the later gas couldn't escape so easily, so allowing the chamber pressures to climb and the projectile to move. That can't be right, though, because then (I think) there would have been recoil. So, presumably, what actually happens is that a bucketload of hot gas flies out the back and you get a simple (Newtonian) explosion - mass times velocity of the gas equals mass times velocity of the projectile, the weapon stays still. Now if that's true why was it so bloody heavy? Presumably you would need a load of steel around the round itself to resist the pressure caused by the rapid expansion of gas, but the breech pressure once the projectile starts to move would (presumably) be half of fanny adams. A rifle needs a strong barrel because there is burning propellant causing high pressures progressively accelerating the bullet the whole way up the barrell. But if all you are doing is (essentially) causing an explosion and then directing one bit of it (the projectile) I can't see that you need a uniform thickness on the barrell. Most of the barrell is just there to hold the rifling, no? Similarly if the venturi just protects your legs when prone you could half it, make it of something else lighter, or adopt a different prone firing position. The reason I ask is that you might think that the total propellant to drive a mass a distance must be similar whether you use fast burning stuff in a recoilless rifle or slow burning stuff in a rocket. If we could have a thin (and light) barrell maybe we could have something cheap, effective and accurate for Afghanistan.