The Beretta 92FS appears to be the sidearm carried by the Military Police personnel at Hawkins National Laboratory. The use of the pistol is anachronistic since the M9 pistol didn't enter U.S. military service until 1985, while the show takes place in 1983. It would be more accurate for them to be issued the Colt M1911A1 like the government agents. The majority of these pistols in the series appear to be replicas.
Jonathan Byers (Charlie Heaton) takes his father's Smith & Wesson Model 10 when he and Nancy Wheeler (Natalia Dyer) set out to find the monster in Myrkwood in "Chapter Five: The Flea and the Acrobat" (S01E05).
Heckler & Koch MP5Ks with Navy trigger packs, sometimes fitted with a flashlight, are used by government agents and MPs throughout the series. Jim Hopper uses one in "Chapter Eight: The Upside Down" (S01E06).
Heckler & Koch MP5K (Mock MP5K-PDW)
Heckler & Koch MP5Ks with PDW stocks attached are also used by agents. Since the show takes place in 1983, this is considered anachronistic since the MP5K-PDW variant was not developed until 1991. Some of the stocks actually look to be UMP-style stocks, making them even more anachronistic.
Rifles
Mossberg 42M
When Will is running from the Monster in S01E01, he takes shelter in the shed and loads up what appears to be a Mossberg 144 LS that is missing the front portion of the Mannlicher stock.