The SIG-Sauer P228 is the main sidearm of nearly all the NCIS agents in the series including Special Agents Leroy Jethro Gibbs (Mark Harmon), Anthony DiNozzo (Michael Weatherly), Timothy McGee (Sean Murray), and Ziva David (Cote de Pablo). NCIS Special Agents Gayle Levin (Alimi Ballard) and Simon Cade (Matthew Willig) draw their P228's in Season 8's "Swan Song" (S8E23).
SIG-Sauer P230
CGIS Special Agent Abigail Borin (Diane Neal) carries a SIG-Sauer P230 as her off-duty weapon in Season 8's "Ships in the Night". She is seen pulling it from her purse at the episode's beginning.
Beretta 92FS
The Beretta 92FS (used in lieu of the Beretta M9) appears as the issued sidearm of various military personnel and in the hands of various characters throughout the series. In Season 8's "Enemies Domestic", during his flashbacks to his first mission in Amsterdam, Director Vance is seen with one. It's also seen in "Defiance" where a US Marine uses it to confront a suicide bomber.
Colt Officer's Enhanced Mark IV
What appears to be a Colt Officer's Enhanced Mark IV Series 80 is the sidearm of Baltimore P.D. Detective Anthony DiNozzo (Michael Weatherly) in Season 8's "Baltimore". The handgun appears to have a skeletal hammer and Novak sights.
Glock 17
The Glock 17 is another frequently-used handgun in the series. Marine Sergeant Heather Dempsey (Abby Brammell) uses a Glock to shoot an intruder in "Short Fuse" (S8E02). Mossad officers Malachi Ben-Gidon (T.J. Ramini), Amit Hadar (Arnold Vosloo) and Liat Tuvia (Sarai Givaty) carry Glock 17's as their sidearms when they accompany Eli David to Washington D.C. NCIS Director Vance (Rocky Carroll) uses a Glock 17 taken from Hadar in Season 8's "Enemies Domestic".
Glock 19
The Glock 19 is a frequent weapon seen in the series. A terrorist attempts to dual-wield Glock 19 pistols in Season 8's "Enemies Foreign" after using a Spike missle launcher.
Glock 26
NCIS Special Agent E.J. Barrett (Sarah Jane Morris) can be seen carrying a subcompact Glock 26 as her sidearm in Season 8's "Swan Song".
Magnum Research Micro Eagle
A Magnum Research Micro Eagle is used by Paloma Reynosa (Jacqueline Obradors) in Season 8's premiere "The Spider and the Fly".
Smith & Wesson SW1911PD
Mike Franks (Muse Watson) carries a Smith & Wesson SW1911PD with the full 5" barrel as his sidearm. What differentiates the PD model are the front serrations on the slide that's similar to those in such handguns as the Kimber. The pistol also has a desert tan Crimson Trace grip-mounted laser sight. The SW1911PD is seen in the Season 8 premiere "The Spider and the Fly". Franks also pulls it in Season 8's "Swan Song" before it was taken by the Port-to-Port Killer Jonas Cobb (Kerr Smith). Cobb is seen brandishing it in Season 8's finale "Pyramid".
Smith & Wesson 5906
A Smith & Wesson 5906 is pulled by a police officer in Season 8's "Kill Screen".
Taurus PT945
A Taurus PT945 is seen in the hands of an intruder that is shot by Marine Sergeant Heather Dempsey (Abby Brammell) in Season 8's "Short Fuse". The weapon seems to be the same used on the set of U.S. Marshals because the serial number is filed off. Also the Taurus PT945 first series is not easily available with the silver hammer, in fact was produced almost exclusively with black hammer.
Tokarev TT-33
A Tokarev TT-33 pistol is seen in photos in "A Man Walks into a Bar" as a murder weapon.
Walther P99
In Season 8's "Baltimore", a Walther P99 is specifically identified as the weapon of a suspect.
Revolvers
Smith & Wesson Model 19
What appears to be a Smith & Wesson Model 19 is used by a murder victim's brother to intimidate her murderer in "False Witness".
Ziva carries a Smith & Wesson Model 340PD as her backup gun, which she carries in an ankle holster. She is seen briefly handling it before going undercover in "Dead Air" (S8E05).
Webley Mk IV
A Webley Mk IV revolver was used by CIA station chief Loretta Tennison in "Royals & Loyals" (S8E04).
Submachine Guns
Beretta M12
Alejandro Rivera uses a Beretta M12 to shoot up the safe house in "The Spider and the Fly" (S8E01).
Rifles
AKM
The AKM is a frequent weapon used by terrorists and suspects in the series.
AKMS
The AKMS rifle is used by various other characters in the series.
Colt M4A1 Carbine
The M4A1 carbine can be seen used by various US Military personnel and gunmen throughout the series.
Remington 7400
A Remington 7400 is used by a suspect in to murder a Navy commander and two radio station employees in "Dead Air" (S8E05).
SIG SG 551
Reynosa Cartel soldiers use SIG SG 551 assault rifles to shoot up the country store owned by Jackson Gibbs (Ralph Waite) in Season 8's premiere "The Spider and the Fly" (S8E01). Dual SIG SG 551 rifles are used to act as a machine gun post before Liat and Gibbs shoot out its control mechanism in "Enemies Foreign" (S8E09).
Winchester 1873
A Winchester Model 1873 rifle is owned by Jackson Gibbs (Ralph Waite), who keeps it loaded on the wall. In Season 7's "Faith", Jackson mentions that he had to use his Winchester to kill someone trying to rob his store. He is actually seen holding it on Paloma Reynosa when she confronts him in his store in Season 8's premiere "The Spider and the Fly" (S8E01).
Sniper Rifles
Heckler & Koch SL8
A Heckler & Koch SL8 loaded with blanks is used by Ziva to flush out a kidnapper in Season 8's "Worst Nightmare".
Shotguns
12 Gauge Double Barreled Shotgun
Gibbs swings shut his 12 Gauge Double Barreled Shotgun and hands it to Franks in Season 8's premiere "The Spider and the Fly" (S8E01).
Remington 870 Express
A turkey hunter is seen with a camouflage-painted Remington 870 Express shotgun in "False Witness" (S8E10).
Winchester Model 1200 Ranger
A turkey hunter is seen with a Winchester Model 1200 Ranger shotgun in "False Witness" (S8E10).
Machine Guns
M240 Machine Gun
What appears to be a rigged M240 Machine Gun is discovered by McGee and Malachi during a routine security check to locate potential security threats against Eli David in "Enemies Foreign" (S8E08).
Heavy Weapons
M3 Carl Gustav Launcher
A terrorist is seen using a rocket launcher that appears to be mocked up from an M3 Carl Gustav Recoilless Riflein an attempt to kill Eli David (Michael Nouri) in "Enemies Foreign" (S8E08). It resembles the mock-up launcher used in the series NCIS: Los Angeles and Chuck.
Other
ASP Red Gun (SIG-Sauer P226R)
Plastic replicas of SIG-Sauer P226's are used by Gibbs, Tony and Mossad officers Malachi Ben-Gidon (T.J. Ramini) and Liat Tuvia (Sarai Givaty) during a training exercise in "Enemies Foreign" (S8E08). The "guns" have laser aiming modules attached to their rails that appear to be activated when Gibbs pulls the trigger. Interestingly ASP replicas are solid pieces of plastic, the triggers don't move.
Firearms Cache
Several pistols and other small arms are found by McGee in "A Man Walks into a Bar" (S8E14).