The reason why the drop tubes are curved seems to be to prevent direct contact with the primer of the cartridges below. Even though the risk of accidental detonation is slight due to the low kinetic energy of the next cartridge striking the primer of the one below it, if the total weight of several of them were resting directly on the primer of the bottom cartridge when the next one lands on them it could cause one to discharge setting off a chain reaction of all in the tube