The mortar has a base 1.5 meter radius on the explosion… I see that heavy ordnance and the big one both have 1.5 meter radius on their tooltip with it scaling in heavy ordnance case… Does this mean if you max heavy ordnance and get a point in the big one that the mortar shots aoe are 4.5 meter instead of 1.5 ?
I tried it in game but honestly I didn’t notice any difference and am trying to figure out exactly what the 1.5 is.
Certainly looks confusing on paper.
“Fire Mortar” has a 1.5 meter target area but an innate blast explosion of 3 meters. Upgrading Heavy Ordnance to max values increases the blast radius from 3 meters to 4.5 meters(in my case 4.6 meters 'cause I am past the cap on Heavy Ordnance)
“The Big One” does not appear to be affected by investing into Heavy Ordnance. It’s always going to be a 1.5 meter target area with a 10 meter blast radius.
Yea I really wish that things that increase range like that or duration showed up on the main skills tool tip… I know Blood pox and Black death have something similar but it’s much easier to tell what happens as the 3 seconds increases the duration from 5 to 8 seconds.
I’ve made so many characters and just never really bothered with mortar because I’ve heard nothing but bad things about it… Decided I would try a vindictive flame tank with mortar for support fire… so far honestly not seeing why it’s rated so low as the damage is really good if you can hold the target still. I mean if it’s a kite or sorc build I see it being bad since you end up kiting.
I just didn’t have any proper way to test if it was upping the range of the explosion as the training dummies are to close together to tell if it’s getting increased or not and the mobs usually run around so fast it’s hard to get a accurate measurement.
So if I do 16-12-1 into the mortar line it will have 4.5 meters on the aoe… That’s not that bad though that range is still kinda small… Glad I am tanky and can use pet attack to force it to blow stuff down that get near me.
Afaik with enough points invested, you can put down 2 or 3 mortar traps, and 4,5 meters is pretty much ingame. Just put your mortars down down and kite in a circle around them maybe? Should be fun. 
Or use something like grasping vines with entangled to keep them in a spot