Based on my experience with 18.5" 243 and 308's, I'd be betting on a flamethrower with a 16.5" spout. I prefer 20", give or take an inch, when using the 7mm-08 or 308.
I had a 308 Savage Scout cut to 17". It looked like a 202 Flash when you touched it off, IE a BALL of fire.
