eh, some polymer or whatever could be invented tomorrow and that would invalidate your argument. but for now you are right, which is why i did post 54. you know, bamboo were the first firearm barrels. is it just the barrel that has to bear the force of the explosion? if so i could also see a scenario where there becomes a market for just the barrels, with the rest being printed.
Recoil impulse would still have to be dealt with. Take a look at flintlock and caplock muzzleloaders, the barrel is a tube with one end closed, except for a small hole at the breech end for the ignition process, the open end being the muzzle. The breech end, which is closest to your face, has to withstand the ignition process. Then take a look at single shot cartridge rifles, be they rolling block, falling block, or break open. Look at single and double barrel break open shotguns, lots of metal behind the barrel(s). Breech pressure and bolt thrust cannot be ignored. Even the simplest blowback semiauto handguns still have enough metal to withstand the bolt thrust upon ignition to delay the opening of the action until combustion is safely completed, and then the action opens, a spent cartridge case is ejected, and a new cartridge is stripped from the magazine and chambered. The physics of the whole process is very difficult to get around.