Somewhat pedantical quibble, really just because I find it interesting: It's not exactly limited by barrel length. We can make faster burning, higher powered propellants, which you can get the full energy out of with a shorter barrel. The reason we don't is because that means you have a higher pressure inside the chamber and, even if your gun doesn't explode, you face more erosion from use. Your metallurgy ends up being the limiting factor, as it's all about how strong you can make your chamber. I just think it's cool because guns are a great example of how inter-related technologies are and how everything depends on everything else. Take a design for a machinegun back to the Napoleonic era and it will be worthless because without smokeless powder it will jam and clog after a couple rounds. Take back a formula for smokeless powder and it will be worthless because you don't know how to make brass cartridges. Try to make brass cartridges and you'll find you lack the precision tooling, and so on.