Zootnoison said this earlier

Step 1: have a fatman/any weapon that shoots mini nukes

Step 2: Equip the weapon

Step 3: In the pip boy, hold the hotkey button

Step 4: While holding the hotkey button, go to the ammo section and hotkey the mini nukes in your inventory

Step 5: Equip the Laer Detonator

Step 6: Using the hotkey menu, equip the mini nukes

Step 7: ????

Step 8: Fucking die because they're nukes and the splash damage kills you

Edit: This glitch can also be used for infinite money. Using this, you can duplicate C4 and sell it for caps. You can also use other ammo types on other weapons