Normally, you can only attach a select number of mods to any given weapon in Fallout 4. This PC mod, however, gets rid of all those restrictions -- which means that you can create completely ludicrous (yet amazing) weapons that would normally be impossible in-game.
Seriously, this is some Borderlands-type shit right here. The mod in question is "Any Mod Any Weapon" by LucasGod. "[The mod is] not balanced, not immersive, and might even affect the non-unique weapons carried by enemies you find in the wild or on stores," LucasGod warns. "But it's really fun!"
Wanna mash together an Alien Blaster with a Fat Man? Do it. Wanna make a triple-barreled electrified sniper-scoped mini-gun? Go for it. Wanna make a target-seeking atomic bomb weapon that shoots 10 mini-nukes into the air? WHY NOT. Your creativity is the only limit here, as evidenced by this Tyrannicon video showcasing the mod in question:
Just...wow. You can download the "Any Mod Any Weapon" PC mod here.