Complex sentence: The reasoning is that the rimless case will not lock together in a double-column magazine, causing a malfunction, whereas an improperly built.38 Super firearm can, as the rimmed cases can lock together in the magazine, causing a jam.
Split into simpler sentences: The reasoning is that the rimless case will not lock together with other rounds in a double-column magazine, causing a malfunction. Whereas rimmed cases can sometimes lock together in the magazine, causing a jam.