If you're going to make a game set in WW2, you model real WW2 weapons.
If your game is set anywhere from 1990 to 2050, and you're trying to model real-world combat situations (with varying degrees of accuracy), then you'll have to model real world firearms. Due to the durability of firearms and the essentially mature technology, you could expect current technology and models to be used for decades. Consider the 1911 pistol for example: that's not a just a model number, that's the year it was introduced. It's also the most common handgun used by serious competitors today.
Savvy gamers today just aren't going to buy it if their High Intensity Combat Operative character in the game is deploying with Generic Intermediate Caliber Select Fire Rifle firing the combat tested 5.44x40mm Solid Lead to Ashcanistan to fight the nefarious Ethnically and Ideologically Unidentifiable Terrorist Organization. They want their DEVGRU to drop out of a Lockheed C-130J into Timbuktu carrying a Colt M-4 Carbine with a Trijicon ACOG on top so they can put a 5.56mm NATO round into the tuches of a Al Qaeda splinter group that's trying to destroy a UN World Heritage site. (Licensing fees paid for all those trademarks.)
If you want to make stuff up, you've got to set your story a long time ago in a galaxy far far away, or some other equivalent narrative technique to put distance between what the player knows and the game-world contains. You can fake medieval weapons. You can't fake modern fire-arms in present-day settings.