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Odd - I've never not been able to pick up something I didn't already have i don't think. That said it does sort of try to restrict you from being too gung-ho. The guns are pretty weak and completely ineffective against the xenomorph and some androids. A shooter it ain't. Also if you play at harder levels things like the flamethrower for warding off the Xeno quickly becomes less and less effective.
i rage quit that game after i ambushed some people with a length of pipe, and tried to pick up their gun. Game wouldn't let me. Fuck that.