Free Fire is a dark comedy / crime thriller about a group of criminals who meet at an abandoned warehouse to initiate an illegal weapons sale in 1978 Boston. The film is a British production and despite being set in Boston, Massachusetts, was filmed entirely in the U.K. Thus, this explains some of the weapon choices in the film. The film was finished in 2016 and hit several festivals but did not see wide release until April of 2017.
The following weapons were used in the film Free Fire:
Justine (Brie Larson) appears to be armed with a stainless Taurus Model 85 revolver. Since the film takes place in Boston in 1978, this may be an anachronism since the Model 85 wasn't offered in the U.S. market until 1984, however, Taurus did start making a overseas version of this revolver in 1977 when they broke off from Smith & Wesson, so it's possible this is an overseas market pistol (despite not being available to U.S. shooters until the 80s).
Smith & Wesson Model 36
Harry (Jack Reynor) takes a Smith & Wesson Model 36 out of the van and uses it to shoot Stevo (Sam Riley), starting off the firefight. He uses it throughout the rest of the film.
A Smith & Wesson Model 629 is used by Martin (Babou Ceesay). Though it should be a nickel Model 29 (which had been in circulation for years before the year 1978) the stainless 629 was introduced in 1978, which makes it not anachronistic for the film, but only barely.
Rifles/Carbines
Beretta SC-70/223
Several Beretta SC-70/223s are seen in an arms deal, distinguished from the standard AR-70/.233 model by their folding stocks. The rifle is referred to by name by Vernon (Sharlto Copley). The offer of the SC-70/223 is met with disappointment by the IRA members, who were looking to procure M16 assault rifles. This rifle is used by Ord (Armie Hammer) and Chris (Cillian Murphy).
Several characters are seen armed with the M1 Carbine. It is a post-war variant with a perforated metal heat shield and post-war rear sight and bayonet lug.
Guns in the Poster only
Valmet M76F
In the most commonly seen version of the movie poster, Ord (Armie Hammer) is holding a Valmet M76F rifle, though he doesn't use one in the film.
Remington 870
In the movie poster, Vernon (Sharlto Copley) holds a Remington 870 shotgun, but again, he doesn't use one in the film.