These are couple flamethrower weapons I've seen in games which you could use to solve this conundrum:
1. Flame rolls while flying after being fired (classic F weapon from Contra).
2. Flame shot flies while leaving fire trail (Metal slug 1 F weapon).
3. Flame shot explodes on contact (Gunstar Heroes combo of Rapid Gun with Flame).
I personally choose #2 but I share the other two as well for choice.