Hello there,
[Kullervo]'s signature shotgun isn't something amazing, I'll say more... it's terrible shit, supposedly useful and helpful in [Kullervo]'s hands but... he builds combos in the blink of an eye even without this weapon, I mean... it's "good" shotgun for pure raw damage (up to SP Butcher lol), but for everything else... it's a waste of your formas. And yes, I tried HM build, still shitty.
Also, try this build purely for its gimmick... to passively build up a combo for your melee ;)

You can use either Dexterity for more Combo Duration, or Merciless for additional +30% Reload Speed :)
It's funny to use an actual firearm to boost your melee :D
Enjoy!
THeMooN85