Complex sentence: The extremely high velocities used by the Paris Gun to attain the stratospheric heights necessary for extreme range caused enormous wear of the barrel, so much so that the shells had to be made in gradually increasing diameter to suit the rate of wear.
Split into simpler sentences: The extremely high velocities used by the Paris Gun to attain the stratospheric heights necessary for extreme range caused enormous wear of the barrel. So much so that the shells had to be made in gradually increasing diameter to suit the rate of wear.