These pistols are carried by NCOs at the Tomb of the Unknown Soldier
Many military members are familiar with the sight of a shift change at Arlington National Cemetery's Tomb of the Unknown Soldier. Only the U.S. Army's finest can join the Old Guard and walk the carpet as a tomb sentinel, so the highlight of any visit to Arlington is catching the Changing of the Guard, where the guard's M-14 rifle is famously inspected during the ceremony. What you might not notice is the duty NCO's sidearm pistols, holstered but clearly ready for use. This weapon is as clean as the rifle the NCO inspects, with one important difference for the guards.
The M-14 rifles used by the Tomb Sentinels are fully functional, the Old Guard says. While the unit would not discuss further security measures due to the sensitive nature of what they do, it's clear the rifle isn't loaded when it's carried by the men walking the line in front of the Tomb. An M-14 with a magazine is distinctly different than one without. Furthermore, when the rifle is inspected during the Changing of the Guard, the inspection would eject a round from the rifle, were there a round in the chamber.
No one really knows if there are live rounds in the nearby tent or another means for the sentinels to defend themselves in case of an active shooter. But the NCOs are packing pistols.
When an NCO of the Old Guard attends to the Changing of the Guard at the Tomb of the Unknown Soldier, the NCO is equipped with a custom, U.S. Army-issued weapon, the Sig-Sauer M17. The weapon was built by the gunmaker specifically for the Tomb Sentinels and comes with a number of beautiful features. There are only four like them ever created, and all are carried exclusively by NCOs in the Old Guard.
The hardwood in the grip of these special pistols comes from the deck of the USS Olympia, a cruiser first laid in 1895 and seeing service in the Spanish-American War and World War I. Marble from the Tomb of the Unknown Soldier is superheated, converted into glass, and added to the weapon's sights, making for one of the most unique weapons created for the military anywhere.
Since things are so tight at the Pentagon in terms of operational security, it's not known whether the NCOs are carrying ammunition for the sidearms, but since there is a magazine in the weapon, they certainly could be. After the 2014 shootings at Canada's Tomb of the Unknown Soldier and subsequent spree on Ottawa's Parliament Hill, they certainly should be.