After poking around on NexusMods, I realize that Cheat Terminal exists and does this with just a holotape.  I also know that Companion Infinite Ammo covers the companion aspect unless the weapon is flagged with "NPCs use ammo".  If for some reason you do not like/use the aforementioned mods then continue reading I guess.

This mod adds an Infinite Ammo Battlecoat, which is just Maxson's coat with the ability to give infinite ammo while equipped.  The only requirement being you actually possess at least one round of the ammo type in your inventory.  Entering Power Armor while wearing the Infinite Ammo Battlecoat will retain its effect.

For those of you who may want to use your favorite custom armors, there is a ring you can use to apply the effect to it.  Drop the armor on the ground, face it and 'Attempt Wizardy'.  The armor has to have the Legendary attach point in order for it to work.

There is an option to add an 'Aura' that affects non hostile actors.  Useful if you don't want your companion to just be a pack mule, but the aura currently does not work when you enter Power Armor.

Both of the items are crafted at the chemistry workbench.

The aura is added at an armor workbench.

Technical Junk for Nerds
If you apply infinite ammo to a custom armor that uses a custom power armor frame (vault suit for example), you WILL NOT receive the infinite ammo effect when you enter power armor.

Equipping infinite ammo armor on a companion may randomly stop working.  The most reliable method is to wear the armor and use the Aura of Infinite Ammo.

You can apply the Infinite Ammo effect to Power Armor pieces if you want to use the Infinite Ammo Aura.

Does not work on the Junk Jet as it uses scrap items and not ammunition.

Updates

Version 1.1 - The Infinite Ammo effect gives the following buffs

• 50% damage bonus (another 50% damage bonus against Legendary Enemies)
• 40% damage reduction
• less movement penalty with heavy weapons
  

The Aura effect no longer alerts enemies.