Complex sentence: Modifications to the rifle included shortening the barrel at its breech and resizing the chamber, so that the shorter ogive of the new bullet would not have to jump too far to reach the rifling.
Split into simpler sentences: Modifications to the rifle included shortening the barrel at its breech and recutting the chamber. This was so that the shorter ogive of the new bullet would not have to jump too far to reach the rifling.