The February 23rd patch (1.30) fixed the modding of weapons.
You can still mod your backpack space and gun proficiency. You won't be able to mod guns outside the guns part list. so if you're modding a revolver, you have to use a revolver barrel.You can mix certain guns (see below). For unique parts like "The Dove barrel" or "Sledges Shotgun barrel" you have to have the right title, prefix, manufacturer, and custom weapon type.
You can still create guns that are better than you could find in the game.
If something on the gun is not as it would be created by the game, they will be deleted while the save loads. You have to have matching manufacturer for shared parts and manufacturer. Also if you have a "Tediore" manufacturer you cant use a "Hyperion" grip or sight and vise versa.
Manufacturer Material have to match the Manufacturer of the gun.
Titles are still inter-reversible, for example the shredder combat shotgun title on a combat rifle or a sniper.
Parts such as Barrel have to match the Barrel slot, which makes the Action slot useless to all guns except Repeaters and Machine Pistols. For all other guns it must be labeled "None". However, Titles can still go in the Prefix slot and Prefixes in the Title slot, as long as the gun has at least one title. Titles still don't have to match accessory/barrel. This means that even though a "HellFire Bitch", for example, can't happen in the game, it can be constructed and will work.
Hardly any modding works with class mods, but COMs with the manufacturer and material swapped over don't get deleted, they just don't work properly. This can be done to some weapons too, but the game chooses a new manufacturer for the gun. More annoyingly, shields and mods with certain manufacturers that used to work get deleted for no apparent reason*. Shields can't have the really really high capacities any more because the usual way was to spam them with the Pangolin material, which gets them deleted.
It is possible that the manufacturer slot is unmonitored when action, accessory, material etc. are; materials can go in it, and I haven't tested other things. Since the game chooses a manufacturer at random even if there is one in the material slot, I wouldn't be surprised if the manufacturer doesn't need to be specified at all, which might leave its slot free for extra materials.
List of mixable weapons (1st weapon parts onto 2nd weapon type).
Combat Rifle —> Support Machine Gun (Must use Support Machine Gun mag).
Rocket Launcher —> Grenade Launcher.
Repeater —> Machine Pistol (Must use Machine Pistol barrel)*.
Sniper Rifle —> Semi-auto Sniper Rifle (Must use Semi-Auto mag).
Combat Shotgun —> Assault Shotgun (Must use Assault Shotgun mag and body).
I hope you found this useful. For video modding tutorials click here
For the best possible examples of modded weapons Click Here