Screenshot made by Sly Boots

As you will see, the image shows a floating weapon handled by an NPC who is invisible. Without forgetting that when handling our rifle, strange black and gray textures appear that annoy the player, even when aiming with the sight of the weapon.
This certainly bothers a lot and makes one lose the desire to play.

However, everything can be fixed.

So here I am going to show you how to fix this problem.

You will need to run the game first. Once inside the game, follow each of these steps:
You must first enter Options.

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Then enter the Video option.

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Then enter the Basic option.

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And finally once you have entered the Basic option, these 5 options will appear.

I have the options configured in this way, because since my graphics card is low-end, I cannot demand much from it.
If you have high-end graphics cards, you will be able to max out all of these options.
Well, I'm drifting off topic.

To fix the issue where invisible NPCs appear with their weapons and the weird black and gray textures that are annoying on the player's rifle, you will need to change the Directx option:

If you have DirectX 11 enabled, change it to 9.

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If you had DirectX 9 enabled, then change it to 11.

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Whether you choose one or the other, it is recommended to exit the game so that the change has been applied successfully.
Once this is done, run the game again. Start any campaign mission and if all went well, the problem should be gone.

If the guide has helped you solve this problem, don't forget to give it a positive rating as this will help other people to see the guide and thus be able to play this game without having any of these problems.