I believe the RNG crafting excuse is mistranslated, but both possibilities are wrong.
Generally, we are told we cannot ‘craft weapons from scratch’ because it 'does not fit the setting.
Most people believe that this statement means “Weapons cannot be made in W40k.”
That is not true in the lore. While many ‘super tech’ items are difficult to make, or source the ingredients for, and there are indeed relic super weapons that cannot be replicated, I do not think this is the intended meaning of the statement. If it were, it would still be a mistake, as chainswords and lasguns, combat knives and axes, stubbers and auto guns are considered fairly low tech in w40k and are manufactured in very large numbers. Still, I think that is not the meaning of the statement.
Rather, I think it was intended to communicate “In Vermintide, we were making swords and flintlocks - from base components - and that is easy to make… but now we are looking at weapons with many parts, and that is impossible to make from a lump of iron in a blacksmith shop.”
This statement, while it has truth in it, is also wrong in both parts. First, flintlocks and swords are NOT easy to make from iron ingots and wooden stocks, they took highly skilled craftsman to make. As such, the idea that Victor Saltzpyre could take base components into the forge at the castle and emerge with a rapier or a brace of pistols, or especially a repeater pistol, volley crossbow, or a great sword, is silly.
Swords require extremely advanced metal treatment to stand up to the punishment they get. As the sword gets longer and/or thinner, these demands become more extreme. The idea of folding metal which has become so famous by Japanese smiths is actually a purification process used to remove impurities from the iron. While it doesn’t really make a super metal, it does very effectively purify the iron so it can be carbonized to create steel without serious flaws that would lead to it breaking. It is a brilliant technique to get pure iron from an impure source. That is the most famous, but all sword makers in the history of the world have been medium/large operations of highly skilled craftsman, not small ones by village blacksmiths as in movies.
Flint lock firearms also require very high end metal and processes relative to their age to contain the pressure driving the bullet/ball out of the barrel.
Additionally, while I believe that the statement “It doesn’t fit the setting in Darktide, but does in the Vermintide games,” is incorrect because of the complexity of the weapons in Vermintide, it is also incorrect because of how weapons are crafted and the usual facilities we’d expect to find on a warship.
Firstly, I personally have crafted several AR-15 rifles. I did not make the parts, I assembled them. I have also assembled a FAL, a 9mm handgun, and done low end gunsmithing on a few other weapons.
When I want a new AR-15, it’s a lot like building a new computer. You choose your parts, you buy your parts, you do a little checking on compatibility, and then you get all the parts in the mail. You go to a gun store with a federal firearms license to buy the receiver (frame on some weapons) which is the part considered ‘the gun.’ Every part attaches to that part.
Then, at home with all the parts and the receiver, you assemble the parts until you have a complete firearm. It is true I am not taking iron and carbon and chromium and making a barrel myself. I am purchasing a barrel. They run around $200 dollars US for a fairly good barrel, but can be much more.
Here are pictures of ‘parts kits’ without the receiver for an M-16/AR-15 pattern and a L1A1/FAL pattern weapon. Alternately you could buy each part individually for a totally custom weapon. IE, my first AR-15, when I was more ignorant, came with a heavy barrel. Years later I took it apart and swapped out the barrel for a light weight barrel, and the ‘furniture’ (stock, grip, handguard) for 1970s glossy black like in the old Vietnam war movies. Now it’s an A2 upper/lower with an A1 setup, including a pencil barrel with a 1/12 twist to the rifling, on it. Another I made with a barrel from faxon firearms, a lower from Sharps brothers, upper from gibbz arms, etc. Each part was selected to give me what I wanted in weight, balance, barrel length, optics, unusual side mount charging handle, even caliber.
The “hard part” is putting on the barrel and ensuring it and the bolt have the correct spacing, *search term ‘headspacing’ and that’s not too difficult either. Some weapons (AK, CETME) require a large ‘machine press’ to push the barrel in, but the FAL and the AR do not. The FAL barrel screws in, and the AR-15 slides in, and is then fixed with a large nut (called a castle nut) that goes around the barrel. The only special tools needed are a torque wrench, a barrel nut torque wrench attachment, a large mounted clamp, and a way to put the receiver in the clamp securely, (such as a receiver block.)
If I wanted the barrel cut down, or the gas system seriously adjusted, or the barrel rethreaded or anything very difficult, I could take it to a professional who would charge me less the $100 dollars on average for the work.
So, when “Crafting” a gun, you acquire parts, you usually do not make them from scratch. A starship with a large armory supporting a war on a weapons manufacturing plant would have access to a massive quantity of parts to assemble any weapon they wished from. It would have the services of many armorers, (this isn’t exactly a gunsmith, but it’s close enough and I’m long winded anyway.) It would have a large supply of spare parts for the most common weapons, from which a custom weapon could be assembled, such as a sniper or short barrel variant of a default pattern.
Additionally, warships usually have full machine shops. The reason is they are expected to be far from home and the need may arise for a part they do not have in stock, most often for the ship or an airplane on the ship. I was once able to tour the machine shop on the USS JFK, an active duty modern aircraft carrier, but I am including a photo from the machine shop of the USS Midway, an Aircraft carrier laid down during WW2 which is now a museum.
When these machine shops are taken into account, even the idea that the parts cannot be made from scratch on the ship falls apart. Remember these machine shops can make airplane and ship parts TODAY. Gun barrels might or might not be more difficult, as they need to be able to withstand extreme pressure and have the Rifling cut into them, but even that should be within the abilities of the tech priests onboard.
TL:DR
If anything, building a weapon from parts and or even making those parts is MORE REALISTIC on a warship above a planet with factories then it is for Victor to make a rapier or flintlock the castle forge.
Therefore, I submit the idea of ‘crafting’ a ‘new weapon’ of any type less complex then the plasma gun easily fits the setting, and that the “idea that it does not fit the setting” is no more then an excuse not to include it. An excuse that does NOT hold water and should be abandoned.