What new weapons or improvements on old weapons made the Civil War the deadliest war in U. S, history?

Machine guns and repeating rifles

Explanation:

Machine guns were used in the Civil War. Compared to the powder muskets of the revolutionary war, These guns didn't need to be reloaded as often and as a result could inflict more casualties on the opposing side. They were also an all around improvement, with better accuracy then their primitive counterparts.

what was the significance of the plessy vs ferguson ruling and jim crow laws

answer the impact. plessy v. ferguson allowed 'separate but equal, also known as segregation, to become law in the united states