Explanation: This setup is for Gley's Unlimited Ammo. You can swap out the three elemental mods for any of your choice.
For sub-stats, you can select whatever you prefer for the fourth one.
Damage Against Colossus or any elemental damage. I personally prefer Chill for mobbing and I already have my shotgun built so I won't be using it against Colossus. Weapon switch could be good for more utility in fights.
If you want to upgrade further, you can add two more mods with weak point damage and specialize in a single element.
Cheers!