"set on fire threshold" for a weapon sets at what damage roll fire is made. This basically confirms that every tile a separate damage roll is made for an explosion, and some of them don't hit the fire limit.
Harzarding a guess: the zigzag effect is caused by the ice floor tiles being hard to set on fire or something since you're setting those on fire less. Totally wild guess, though.
For some, the threshold is low, so you'll basically always see it with fire bombs/willie pete
Re: shotguns. Honestly they're only uberpowered early game anyways. Plenty of things deal large amounts of damage to softies later by back to school or with high experience gals (ninja stars, e-pulse, slayer (although who even makes slayer bullets) etc). Also what would peasants even use if the shotgun required aiming? XD Besides: The mod follows anime rules more than IRL rules
Edit: nvm on the fire thing, I've no idea. I'm assuming those are hell mortars in which case it should set everything on fire :/