There are 2 types:

• For primary weapon: Akimbo Judge Shotguns
  
• For secondary weapon: The Judge Shotgun

• Weapon Skin = For beauty
  
• Ammunition =For damage, accuracy ...
  
• Barrel Ext= For damage, accuracy, concealment, stability …
  
• Boost= For accuracy, concealment, stability...
  
• Gadget= For flashlight, laser…
  
• Lower Receiver= No comment.
  
• Sight (for single Judge gun only)

Let’s talk about modifying the ammunition a little more
(some may require DLCs)

-000 Buckshot = Damage +15
-Flechette (long range armor piercing): Damage (-3), Accuracy (+8)

-HE Round: Total ammo decreased by a few, Damage +3, Accuracy (-13)
Benefit: Fire one explosive charge that kills or stuns targets. Be careful, this can hurts your teammate.
Example below…

-AP Slug= Total ammo decreased by a few, Damage +15, Accuracy +8
Benefit: Fire a single lead slug that penetrates body armor, enemies, shields and walls. Example below…

-Dragon’s Breath Round: Total ammo decreased by a few, Damage (-6), Accuracy (-4)
Benefit: Fire pellets that go up in sparks and flames. Burns through shields and body armor.
This cannot hurts your teammate, don't worry.
Example below…

Let’s focus on some highlights below such as total ammo, damage, accuracy, stability,concealment and reload time.For the rest are not really important.

Now gonna talking about how to upgrade these weapons more powerful to use as possible. An example will be given for the Akimbo Judge shotguns.


Card No.3 Ambidexterity: Stability penalty the actual number is (-8) ,
and ammo capacity increased by 50% for every akimbo weapons.
Card No.8 Fast and Furious: 5% more damage.



-Ammunition, 000 Buckshot = Damage +15
-Barrel Ext, Donald’s Horizontal Leveller = damage +2, stability +8, concealment (-2)
-Boost for stability +4
and other things else are optional.

The highest this weapon can get...
100 total ammo
206.4 damage x 2
60 stability
and more...

Basically, in a real situation that we can adapt to use another perk deck because hitman perk just increase ammo and reduce a few stability penalty.

Additionally, to use this weapon, we just need to concern about how to not die in the battle so easily despite it's very powerful.

Good:
+High damage
+Modification

Bad:
-Low ammo pickup
-Low magazine capacity (reload frequently)
-Close up enemies to kill with a great damage


Solution:
+Ammo bag+bulletstorm skill > no more run out of ammo and else
+Use suitable perk decks for survivability> Grinder, Stoic and else


There are many things that can increase damage like a critical hit skill and more
but I just don't want to mention about them.

IT JUST WORKS!

Some of calculation may not correct, I tested in game itself and got the result like that.
Just want to make clear, THIS IS NOT A BEST WEAPON to beat all heists and all difficulties. Everybody has their own preference and I respect that too.
Thank you, that's all I can say and feel free to leave a comment for asking or suggesting me because I still finding myself to beat the highest difficulty with all available weapons in this game.


(and sorry for some bad English grammar and whatever, forgive me :D)