What you call what depends on your overall game design. If your going for the realistic shooter then your players would like to see branding but on the other hand if your going a bit more stylized it’s actually better to avoided branding altogether anyways.
To much reality I’ve found tends to split the player base between those who feel you failed to achieved realism and the other half will complain about the lack of ideals as to the traditional “fun” first person shooter.
Just read through the UT sections
If you want realistic then you should be looking into how much licensing of real world elements will cost instead of avoiding it as that is the cost of doing the business of video game development and such costs should be considered as part of you development budget as well as expected returns. Your game might sell a million bucks but consider yourself lucky that you get to keep 25% of that in your pocket.
Overall though the easiest thing about making a game is making the game and with out taking into consideration what it takes to bring a game from concept to completion as a delivered product is where most fail as most will attempt to make their game to much on the cheap.
As for generic names make something up that sounds cool that has no real world counterpart like a Delta 18c 9mm semi auto made by OzoneBG Armories…