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I have run into this problem with both frenzy/berserk barbs I've played. After a little testing the best I could come up with is that sometimes the game will choose the primary weapon based on whichever has the LOWER maximum 2h damage (including elemental).

I could be totally wrong on this and there might be another factor the game uses to pick a primary weapon (lvl req/AR??). But for now my advice would be to look for a weapon with decent damage and something like +1-32 lightning damage so that the max damage goes above the weapon you want to be primary.

Hope this helps, good luck.