Complex sentence: Stored by the military in a "pattern room", a pattern musket served as a reference by which arms maker could make comparisons and take measurements to insure that they could produce firearms that would achieve some level of standardization.
Split into simpler sentences: Stored by the military in a "pattern room". A pattern musket served as a reference by which arms maker could make comparisons and take measurements to insure that they could produce firearms that would achieve some level of standardization.