Well?

Personally, for me it's always been the bolter; back when I was eleven, the idea that there could be a science fiction future where people didn't use laser weapons was staggering in its originality. That these non-laser weapons would basically look like AK-47s was even more amazing, because it felt so much more real. Then to discover that they shoot rockets?

And I know it's not, and has never, had a great in-game profile. I know that mechanically, it's the yardstick by which every other ranged weapon is measured against, making it the most average gun in the setting.

But for fluff, every other weapon in 40K has basically been playing catch-up as far as I'm concerned.