You would be using CoD’s assets, which would make it a derivative work.
Whether that can be proven in court is a different matter.
Viewing post in "Copyright" or "Transformative", "Fair Use"?
Activision will have licenced it's use for their COD games from the manufacturer (Heckler and Kock remain the owner)
Is this a known fact? From what I could dig up, in artistic works like this, that is not needed. In general. All this is case by case decisions.
Trademarks do not really apply, since Activision is not trying to sell guns and HK is not selling video games.
And if patents are violated because there is a depiction of the gun, that puts a lot of questions on the merit of patents and the meric of the gun. The patent was not granted for a video game represenation of the gun, but for the real life gun.
There might be copyright issues, if they simply took renders of promotion material from HK and extracted the 3d model.
The point is, this is not a case of a toy gun manufacturer wanting to create a toy version of that weapon. They make a shooter game with the characters using context appropriate equipment.
Maybe they have an agreement, maybe there is even a 3d render model maker that has such agreements to sell assets. I do not know. But since contemporary weapons are popular in contemporary shooter games, I would assume they do not all reinvent the wheel and make their own assets from scratch.
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You using CoD as a base has it's own problems. Mostly copyright, I guess. Screenshotting is not an excuse for anything. They are neither analogue, nor does it matter how your copy process was done. They own the copyright for that game. The whole game. Not excluding parts of it. That's like saying, since they do not own the sky, a screenshot of the sky out of that game would not be protected. It is the rendering that was done on their engine in their game with their textures. Making an imperfect 1:1 copy of those pixels is copyright issue, in my opinion. Do not forget colors and shades.
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The worst would be, if HK offers assets themselves. If I would own that company, I would. To ensure accurate represenstation of my products in fictional media like video games.
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But here is the kicker. Everyone using your asset would face the above issues. If there are any. I do not know. It is an interesting question.