Hello.
I've got a question about using real weapons' models and names in a free open source videogame (not commercial), which will exist by voluntary donations.
If we create our own weapons' models representing the real guns and name it same as in real life (without manufacturer name), will it count as fair use or what?
I know about modifying names like making AK-47 AS-47 or smth like that, but we want the game to be as close to the reality as possible, so we want to know, are there any ways of doing that.
Thanks in advance.

In short, no, not really. Fair use is a defense to a charge of copyright infringement. It's mostly applicable to, say, news media showing a clip of a movie, during a movie review, without getting a license from the publisher (as a quick example).

That said, calling your weapon something like AS-47 (or something even more "transformative", like KA-74, or something else totally off) tends to help.. and the more transformative you get, the better off you are. (Think: developing and entire backstory around the "KA-74 brand name" in your game would take you farther away from the reality of the AK-74 trademarks/IP, and reduce the likelihood of any legal action against you).

And, of course, any company realistically wouldn't even notice your game until it reaches scale large enough for them to care about the revenue they could make from licensing to you.

(Some notes on me: I have a law degree, but I don't work as a lawyer heh. But in giving you what I know from school and what I've seen in the news and such)

So, it doesn't matter, that game is not commercial?
And talking about trademarks, I have certanly found one for P90, but not for M4, for example, will it make a difference?

First off, whether you work is commercial or not is largely inconsequential as far as fair use is concerned. Infringement is infringement, regardless of if you're making money off it and regardless of if you want to make money off it.

Second, fair use only applies to copyright - it is totally irrelevant to patent and trademark, which are the relevant IP protections here. (The names would be covered by trademark; the physical appearance of the weapon would be covered by design patents.)

tl;dr don't use real products in your game unless you can afford to investigate the legal status of the various IPs involved or risk the legal consequences.

Concluding all said, I guess, that the best way is just to contact manufacturers and ask for their permission.