barrel flexing

In that case I am surprised that they do not just use larger diameter but thinner aliminium tube -- like Cannondale & Klein mountain bikes, which are very stiff. I suppose increased side drag could be an issue with bigger tubes. So, modified approach might be to use a different tube shape to add stiffness, perhaps an oval cross-section as used on many bikes these day -- oriented to reduce side drag. I guess that is similar to what some of these fancy new carbon fibre spearguns do.

Somebody mentioned that routing the rubbers from lower down prevents this flex. I think the black viper has a front hole -- presumably designed for a second rubber band (US-style). If the flex bothered you a lot, perhaps you could switch to using that?

If you are hitting the fish though, why bother?!