In normal circumstances, Fallout 4 only allows you to tack a set number of weapon modifications onto your chosen firearm. The Any Mod Any Weapon mod for Fallout 4 asks: what's stopping you? Imagine if you turned up with, I dunno, an Electrified Triple Barrel Minigun? They'd shit an irradiated brick for sure. And with no crafting restrictions, you could teach those Super Mutant Ancient Behemoths obnoxiously roaming the Commonwealth a serious lesson.
Ever been sat in front of a Weapons Workbench with a tonne of material, wishing you were free to craft whatever the hell you wanted? Sure you have.
