RE Retrofit:
The thing is, the uppers already exist and are produced by a number of companies. It’s an easy swap, the guys are already trained on the platform, the ergonomics, mags, and ammunition don’t change... it runs cooler, runs cleaner, and runs better.
It would seem, on the face of it, to be more cost effective than developing a new firearm to replace it.
And like I said - it works. Look at the AK47 and it’s myriad variants. Piston drive is the way to go.
RE Flamethrower:
I would never, for any reason, want to strap one of those bombs on my back. Every rifle in the area knows who and what you are and what it will do, and will concentrate absolute ballistic HELL on you... those WWII guys had serious brass balls strapping those things on, for sure.
But you are correct - it was/is an awesome and terrifying battlefield weapon. I believe they had them on tanks back in WWII, also.
They actually don’t explode in a big ball of fire when they’re hit. Still, a bullet doesn’t have to make anything on you explode to kill you, and those tanks still make you a big, slow-moving target.
http://tvtropes.org/pmwiki/pmwiki.php/Main/FlamethrowerBackfire