Corebook says you are to use Trade (Armorer) to apply weapon modifications. To remove already installed you are to use Tech-Use, though.

Enemies Within adds a Frontier homeworld, which says "a frontier world character gains a +20 bonus to Tech-Use tests when applying personal weapon modiications".

How to resolve this collision correctly? Should we read the latter as "a frontier world character gains a +20 bonus to Trade (Armourer) tests when applying personal weapon modifications"? Are there rulings that I overlooked while studying the book? What skill should be used to apply a sight on a rifle?

I'm also interested in repairing rules. Repair is mentioned several times in core rulebook, but there are no rules similar to crafting ones; only a paragraph about repairing vehicles. Any thoughts? Any things I didn't notice?

Perhaps Trade (Armorer) is an alternative test to normal Tech-Use tests to apply modifications?

Edit: Nevermind that. A Faq/errata would be nice here. Perhaps simply houserule that the +20 bonus applies to both Tech-Use/Trade(Armorer) skill tests with regards to modifications.

I think I would have it depend on the type of modification, meaning how easy it is to install. Some combinations lend themselves to a "plug and play" kind of installation because the weapon's designers already planned for it, whereas others require a major modification to the weapon itself in order to mount an attachment.

For example, affixing a targeter to a boltgun already equipped with a mount might be Tech-Use, because it involves nothing more than slapping on a sight and connecting a few wires. Anyone with a basic understanding of the technology (= Tech-Use) should be able to do it. The same goes for shortening a combat shotgun into a Compact variant by simply sawing off the barrel, or for slapping a booster charge to a hammer for the quasi-mono upgrade.

On the other hand, affixing a sight to an oldschool firearm that does not have any mount would require some metalworking with proper tools to make sure the scope is properly attached, as you basically need to fashion the missing mount from the ground up. The same goes for trying to rebuild a plasma gun from the ground up to see where you can save some space and make it smaller, or for using high tech equipment to modify a blade's edge into a monomolecular fashion.

Removing a modification could be Tech-Use for both cases, though, because it's always easier to take something apart. Then again, some modifications might actually be more difficult to remove than to install, such as with the aforementioned compact shotgun (have fun exchanging the barrel you just sawed in half!)...

GM call! You could even have it depend on a weapon's quality, with Normal Quality weapons usually requiring the Trade, whereas Good and Best already come better suited for modifications from the start, and Tech-Use is usually sufficient.

Look at the vehicle repair rules for inspiration at having multiple applicable skills for a test. In this case, Trade(Armourer) is the Essential skill and Tech-Use is the skill the character will be using for a bonus. Any bonuses for being a weapons expert or for difficult modifications is GM fiat.

A further question based on this for much the same reason. A combi tool would help if it's Tech Use, but as it's unclear which skill (or if you could use either) would it apply? Logically I think it would make sense even if Trade (Armorer) was the skill. Is there any chance of there being an errata that might clear things up?

The rules quotes in the OP show FFG doesn't even really understand what the difference between tech use and trade skills is supposed to be. Trade as a skill is a holdover from the old game line and should have been discarded when they merged similar skills. The line between these two skills is so blurry that it should just be completely erased. IMO, use tech use for everything like this and pretend trade doesn't exist.

I daresay Trade is a relic of the old Warhammer Fantasy game. I never really found it to make sense for any of the 40K games and there was very little support for it aside from a few extra bits here and there. If I recall correctly, the Inquisitor's Handbook from first edition provided the most support for the Trade skills but even then the skills had very limited applications.

The only examples for any Trades in use I could find in the core rule book are the following:

• Trade (Armourer) to apply weapon modifications (p. 162). Strangely enough to remove the modification you use Tech-Use and not Trade (Armourer).
• Trade (Armourer) and Trade (Technomat) in the section about repairing vehicles (p. 258 ff). And here it seems that both can always be replaced by Tech-Use.

So indeed, it looks like Trades are pretty useless in DH2 ...

Edit: Just saw that both were already mentioned above, sorry. (Should read the whole thread before posting ...)

"Trade crafting often involves fashioning new, unique items of personal design, and thus do not always require detailed plans for the item." (page 32)

"†Note in the case of Tech-Use, characters cannot create items they do not have access to designs for." (page 32)

For me this is the big difference between tech-use and the trade armourer, with the trade you can design your own weapons or modification and aply them, with tech-use you can only create something from a design previously created.