Question/discussion about engineers weapon builds.

Currently 15. The freeze grenade launcher has just been peak satisfaction so far! Also loving the 2 other shotguns. The starter i use flechette rounds to mow through the first few levels. Second shotgun i Haven't modded too much yet and haven't even tried the 4th weapon

What ammo mod would u suggest on the second shotgun for more boss/beefy target dmg? I was thinking explosive slug. Or how do you guys like to build your demo weapons?