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modern warfare is irregular
it could be small scale, think csgo, type if engagements like stalker anomaly (warfare mode) or it could be large scale sized battles of rank upon rank of infantry shooting at eachother in a mass volley
just because volley warfare hasn't been tried in the modern age doesnt mean it isnt possible or feasible to occur.
Think about what could happen in a neo-medieval age (with modern age ak47/m4's). A religious and brutal and savage world where armies have both population (high birth rate) and guns to spare and none of the super advanced technology employed by empires during the classical modern age
Even such industrial output of ww2 with planes and such would be far too much for neo-feudalistic monarchies who are more content in swimming in their wealth than to go into massive debt like the previous by-gone american empire did