User blog comment:Kogath/Monster Hunter X Discussion/@comment-26116038-20150716133152/@comment-25275196-20150717045849

I feel like the bar is used for wyvern fire or something, using wyvern fire will dump the bar in the red, and the gunlance will recharge over time. You could probably use it again while in the orange, but the yellow will be more powerful, and more powerful gunlances would have green and blue and the like.

It could also just be for regular shelling, and shelling drops this bar instead of the sharpness, and you can't shell in the red (like present gunlances, for those who don't bother to test these things)