Complex sentence: Early synchronized guns of the 1915--1917 period had a rate of fire in the region of 400 rounds per minute; at this comparatively leisurely rate of fire a synchronizer can be geared down and rendered fairly reliable.
Split into simpler sentences: Early synchronized guns of the 1915--1917 period had a rate of fire in the region of 400 rounds per minute. At this comparatively leisurely rate of fire a synchronizer can be geared down and rendered fairly reliable.