Complex sentence: The Seven Years War and American War for Independence were two early conflicts in which the modern rifle began to make a significant contribution to warfare due to its advantage in range and accuracy over the smoothbore musket common among professional armies of the mid- 18th century.
Split into simpler sentences: The Seven Years War and American Revolutionary War were two early conflicts in which the modern rifle began to make a significant contribution to warfare. Its accuracy at relatively long range increasingly offered advantages over the smoothbore musket in common use among professional armies of the mid- 18th century.