It may come as a shock, to players that doesn't know it, but you can swap the mods between the guns! This works for both, the mods you've created previously and the mods that are built in the gun you've picked up. The only rule is that the guns must be the same caliber.
How this affects your playthrough? Well, there are two major consequences! Firstly, If you are using the "Gun Nut" perk, than you can swap the mods you've made earlier onto the new guns you will find. And you won't need to create the mods you've created previously again. Secondly, you can play without the "Gun Nut" perk. While it may sound a bit wild the first time you hear it, it is actually one of the coolest way to create new and unique experience for your next playthrough. Once you will be bored with your current approach, or will decide to start the game anew, just try it! It will completely change you gameplay experience. The gun loot you will find will feel much more rewarding. It will no longer be "oh, again this rifle that i will sell for X caps" or "oh, again this pistol that i will scrap for Y material". Instead, each new gun you will find will be perceived like an opportunity to find an upgrade. Such approach will create an excitement.
Here, i have a nice automatic shotgun. This babe has the best mods in each slot. I had been modding this gun for along time. Had been raising it like a child, from the low level, non-upgraded gun I’ve found to a high-tech, kick-ass automatic shotgun! The shotgun had been with me from the rank 1 "Gun Nut" perk times till now!
Recently, I’ve found this incredible shotgun. It does additional radiation damage. I know it will do more damage, compared to my current shotgun, if I apply latest mods to it. Creating them again is costly.
So, what I do here is I take my high-tech shotgun and put back generic mods on it. This way, all the mods that were previously on a gun will go in the inventory. And, i can install them to my newly found legendary shotgun!
Such approach allows me to spend almost no resources, as generic mods are very cheap resource wise, and swap the latest mods to the new gun I’ve found! I just have to select the mod I’ve just un-installed in the mods menu, and it will be automatically installed in the new gun. No resources are needed! Because, you are installing the mod from the inventory! Just remember that the guns have to be same caliber for this trick to work.
Last thing i wanted to mention is that above you see the images of me swapping the mods that I’ve crafted throughout the "Gun Nut" perk. However, as I’ve mentioned before, you can do this even without the perk. And i have a separate character. Actually, i have a zoo of them, about 6 or 7. But the one I’m talking about, his combat is purely based around the pistols. But, I’ve never skilled the perk for modding the guns. I've had ton of fun playing with such approach, as well as tons of mods and modding.