According to a Q&A at the film's premiere, the VSS Vintorez used by Crow was an authentic rifle converted to fire blanks rather than an airsoft replica.
The actors spent 3 days before shooting to learn proper weapons handling and military tactics like breaching and room clearing.
Several scenes of the facility featured in the film were shot at an abandoned nuclear power plant featured in a video from writer/producer Isaiah Nichols (aka Wendigoon)
The acronym "S.T.A.L.K.E.R" stands for "Scavenger Trespasser, Adventurer, Loner, Killer, Explorer, Robber", and denotes the types of people that explore The Zone.
The film is based on the S.T.A.L.K.E.R video game series, which itself was inspired by the film Stalker (1979), which predates the Chernobyl disaster by several years.