This build is simple: 66% chance to Crit for 3.5x damage, plus a 5.3x damage modifier to weak points, all on top of 220k Damage-Per-Bullet, and you have a Colossi-Deleting magic wand in the shape of a bolt-action sniper rifle. Swap out Expand Weapon Charge for Toxic Enhancement whenever you are fighting a Colossi that's weak to Toxic.