Is it possible to upgrade your companion's primary/default weapon?

I enjoy having Nick Valentine as my companion, but his pipe revolver pistol looks like crap. I have quite a bit of resources, is it possible for me to get his pipe pistol and upgrade it?

I know I can give him other modded weapons, but he has infinite ammo for his revolver, so that's what I want to focus on.

In the vanilla game, no, you are not able to replace or modify a companion's default weapon. There are multiple discussions about this online, like this one:

There's no way to get their default weapon. Even if you equip another one on them their default one never appears in the inventory.

Here is another:

No you cant upgrade the default one. Yes you can give them a better one. Put a weapon in their inventorwand press triangle on their side of the screen to equip it. For guns they need ammo though.

However, if you are on PC, there are mods to change this behavior, however, such as this one.