High estimates for atomic bombs are 230 000 Casualties (150k for Hiroshima and 80k for Nagasaki).

The American civil war however was the first war in the occidental culture that saw more death due to weapons than from illness and hunger. This makes counting death toll by weapons very hazardous.

The mongol conquest have an estimated death toll of 40 to 80 Millions. Although the bow was not the only weapon they used, if only 5% of the victims result from bow, that would top the Nuclear weapons casualty with one theater only.

About crossbow casualty however that is much more difficult to appreciate.