Complex sentence: Industry had a difficult time reproducing parts according to the original blueprints, so models that were being procured were mechanically unreliable and mixed with the inventory of working spares, resulting in using a mixed batch of working and unreliable weapons.
Split into simpler sentences: Industry had a difficult time reproducing parts according to the original blueprints, so models that were being procured were mechanically unreliable and mixed with the inventory of working spares. This resulted in using a mixed batch of working and unreliable weapons.