Is there a way to hold a flashlight in the offhand if you are using a sidearm or is the headlamp required?
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you cannot do that(Yet) but several weapons come with a flashlight and several head armors (Such as the ballistic mask helm thingy) also has a headlamp. though i strongly reccomend you aquire the nade launcher and use the flare rounds as they glow for a pretty long time before going out and the rounds themselves are not hard to come by.