FLB was initially a feature designed to give out useful freebies to nonspenders that would not hurt the cash shop sales. So it it highly unlikely that anything "epic" will be in the lootbox so as to prevent people from farming it.
As far as the guns go, I'm assuming you mean WolfTeam? And if so, there are guns in the FLB and some of them are pretty decent. Of course they're not going to be uber epic, and why should they be? "Samples" are given for free at the bakery stand, not "whole cakes"
Oh, if you're requesting something particular for a FLB, please notate the game you are hailing from.