Like all federal agencies in the United States, Special Agents associated with the NCIS agency are required to carry firearms with them at all times as a means of personal protection or in case they are attacked by potential suspects who may be armed if it's in the field or in the office where they work.
Each agent of the NCIS Washington D.C. field office carries a SIG-Sauer P228 which is presumably the standard-issued weapon for all NCIS agents assigned to D.C.
The D.C.'s field office counterpart, the NCIS: Los Angeles based Office of Special Projects originally carried the SIG-Sauer P228 before switching to the SIG-Sauer P229.
Nearly all the agents at OSP carry the new SIG-Sauer P229 except for Operations Manager Henrietta Lange and Intelligence Analyst Nell Jones, whose weapon of choice is Glock 26. Tech Operator Eric Beale don't carry a gun at all, purely because Eric himself has expressed no desire to become a field agent.
Marty Deeks who is a Detective with the Los Angeles Police Detective and also the NCIS/LAPD Liaison Officer is the only other person not to wield the SIG-Sauer P229, instead choosing to carry a Smith and Wesson 5944 as his main weapon due to the fact that he's still with the LAPD- Los Angeles Police Department and not NCIS.
Whenever their guns aren't being used, the agents carry their weapons in holsters that are on their hips which also means that they're ready to grab them at a moment's notice and also at the first sign of possible trouble.
The SIG-Sauer was seen for the first time during the JAG Season 8/NCIS backdoor pilot episode, Meltdown (episode) where NCIS Special Agents Anthony DiNozzo and Vivian Blackadder were seen wielding SIG-Sauers during the anti-terrorist operation in Rota, Spain.
Gibbs later borrowed a SIG-Sauer from the armory on board on Air Force One to shoot and kill journalist/terrorist, Leonard Rish who was planning on assassinating the President of the United States.
In Forced Entry (episode), Kate wielded her SIG-Sauer while confronting a serial killing team.
While on protection detail during Probie (episode), Gibbs cautiously drew his SIG-Sauer after hearing gunshots coming from an alleyway.
In Suspicion (episode), both Gibbs and Ziva drew their SIG-Sauers to confront a suspect.
In Judgment Day Part 1 (episode), while entering the abandoned diner, Ziva drew her SIG-Sauer while examining the area for any hostile threats.
Tony and Ziva drew their guns as they prepared to gain access to the house of a private investigator.
After confronting and disarming a suspect during Thirst (episode), McGee was seen using his gun to stop the suspect from drawing a gun of his own.
During a gunfight against Jason Paul Dean and his men, DiNozzo and McGee were seen using their SIG-Sauers against the group.