Another concern is that you're liable to infringe on a lot of copyright and trademarks.
Only trademark, and at least in the US it is fair use to use a trademark nominatively -- that is, to identify the brand. So LCS includes names like "MP5" and "AR-15" and "AK-47", but it uses them
in reference to the trademarked brand. It's like, you can sell an AK-47, and you can call it that, but you can't call your home-made knockoff an AK-47. It would be a genuine infraction if the game were implying through the use of MP5 that it was sponsored by Heckler & Koch, or trying to sell a different product under the name AK-47, but that's not the case; we're only using the name to say "In the game's fiction, you have the option of buying a MP5 SMG." Intuitively, that doesn't
feel wrong, and at least in the US, it's legally protected.
It's like an author writing about a guy who works in a Coca-Cola factory -- even if it paints Coca-Cola in a bad light, even if it's fictional, even if the author could have written the story about a fictional soda brand, it's not inappropriate because it's using the Coca-Cola trademark
to identify the actual brand by name. Now, if the title of the book were done up to look like the Coca-Cola branding and gave the impression that the book were sponsored by Coca-Cola, that would be different. But we're not doing anything like that; we're just naming some in-game guns as explicit references to the real-world guns by the same name. That is using a trademark someone else owns, but it's using it correctly, respectfully, and in a way that is consistent with the law.