Complex sentence: Owing to this inaccuracy, officers did not expect musketeers to aim at specific targets, rather, early military engagements involved a line of soldiers simply pointing their muskets in the general direction of the enemy and delivering a mass of musket balls into the enemy line.
Split into simpler sentences: Owing to this inaccuracy, officers did not expect musketeers to aim at specific targets. Rather, they had the objective of delivering a mass of musket balls into the enemy line.