Detective Lieutenant John McClane's (Bruce Willis) main sidearm throughout the film is a SIG-Sauer P220R (nowadays, this model is simply known as the standard P220). During the confrontation with Mai Linh (Maggie Q) in the power plant, she takes the P220 and holds it on Matt Farrel (Justin Long). After McClane hits her with an SUV, she loses the gun and it is never seen again; McClane later takes a Beretta Px4 Storm from Russo (Yorgo Constantine), which becomes his weapon for the rest of the movie.
Beretta Px4 Storm
The terrorist Russo (Yorgo Constantine) in the film is seen carrying a Beretta Px4 Storm in several scenes, both with and without a sound suppressor. After killing him, John McClane (Bruce Willis) takes it and uses it in the second half of the film. During the final confrontation with Thomas Gabriel (Timothy Olyphant), McClane drops the pistol, and it is then used by Matt Farrell (Justin Long) to kill Emerson (Edoardo Costa) with.
Beretta 92FS
One of the terrorists is seen using a Beretta 92FS to gun down the hackers in the semi-truck, including Casper (Andrew Friedman). An interesting note, the particular Beretta used in the film was the same prop gun used by Bruce Willis in the second and third Die Hard films. Call it a "cameo" role, if you will.
Beretta 92SB
A patrolman in Washington D.C. is seen briefly holding a Beretta 92SB, noted by its rounded trigger guard and ambidextrous decocking safety.
Many of the terrorists are seen using Heckler & Koch UMP-45s throughout the film. They are all fitted with C-More red-dot sights, Surefire M900A vertical grip/flashlight combinations, and flash hiders. McClane (Bruce Willis) commandeers one after the shootout at Matt Farrell's (Justin Long) apartment, but never uses it.
Heckler & Koch G36C
Another weapon used by the terrorists are Heckler & Koch G36C. Like the UMPs, they have C-More sights and tactical flashlights mounted on them.
M4A1 Carbine
M4A1 Carbines are used by the FBI agents and security guards at the power plant, being fitted with flip-up rear sights, M68 Aimpoint scopes, AN/PEQ-2 lasers, and Surefire M900 weaponlight foregrips.
Jericho 941
Thomas Gabriel (Timothy Olyphant) uses a stainless steel Jericho 941 as his sidearm, which he holds on Lucy McClane (Mary Elizabeth Winstead) in the final confrontation and uses to shoot Matthew Farrell (Justin Long) in the leg. Later, he sticks the muzzle of the Jericho into John McClane's (Bruce Willis) gunshot wound, so McClane simply forces him to pull the trigger, causing him to shoot through his pre-existing wound and into Gabriel.
Glock 22
FBI agents and some Washington D.C. patrolmen are seen using 3rd Generation Glock 22 pistols. They are noted as such by their muzzle diameters and by the fact the FBI actually carries G22s in real life. John McClane (Bruce Willis) at one point takes a Glock 22 off of one of a terrorist disguised as an FBI agent and uses it to kill him.
Mark 12 Mod 0 SPR
The gun used by the terrorist snipers, first in the shootout at Matt Farell's (Justin Long) apartment, and later during chase scene in Washington D.C., is the Mark 12 Mod 0 SPR (Special Purpose Rifle). This is a little-known AR-15 derivative developed by Crane and is used by U.S. SOCOM personnel in Iraq, Afghanistan, and other hot-spots globally. The Mod 0 uses a PRI carbon fiber handguard where the Mod 1 uses either a Knight Armament or LaRue Tactical free-floating aluminum handguard. The weapons in the film are fitted with suppressors, brass catchers, Surefire M900 weaponlight foregrips, as well as AN/PEQ-2 IR designators and are fired on automatic (which suggests that the terrorists simply took the upper receivers/barrels and slapped them on full-auto lowers).
Heckler & Koch P2000SK
Mai Lihn (Maggie Q) uses a suppressed Heckler & Koch P2000SK when she and some of the terrorists infiltrate the power plant.
Heckler & Koch USP Tactical
During the scene at the power plant, McClane (Bruce Willis) runs into a terrorist who is armed with a Heckler & Koch USP Tactical fitted with a KAC suppressor. McClane fires his SIG-Sauer P220R and the terrorist fires the USP Tactical simultaneously over each other's heads before knocking each other's guns away. McClane eventually gets a Glock 22 out of the terrorist's holster and uses it to kill him.
SIG-Sauer P226R
One of the terrorists at the power plant is seen using a SIG-Sauer P226R fitted with a suppressor. When the terrorist is shoved into the elevator shaft by Matthew Farrell (Justin Long), his gun falls into the SUV lodged in the shaft. Mai Lihn (Maggie Q) tries to use the gun to kill John McClane (Bruce Willis), but the car falls to the bottom of the shaft before she can fire off a shot.
Heckler & Koch MP5
A few of the FBI agents in Washington are seen carring MP5s.
Heckler & Koch MP7A1
Another gun used by Thomas Gabriel (Timothy Olyphant) is the Heckler & Koch MP7A1 using one to kill some of his associates, and later Gabriel's henchman Emerson (Edoardo Costa) uses a Heckler & Koch MP7A1 to wound McClane moments before the rescue of his daughter Lucy (Mary Elizabeth Winstead). Later, Lucy knocks the MP7 out of Emerson's hands and tries to kick it to McClane, but Gabriel intercepts the weapon before it reaches McClane.
Heckler & Koch USP Compact
Emerson's (Edoardo Costa's) sidearm is a Heckler & Koch USP Compact which Lucy grabs after she knocks the MP7A1 out of his hands and shoots him in the foot with it, but he restrains her and holds it on her.