This Mk19 has a rather interesting sight mounted. I do not know the make or model for sure, but perhaps a AN/PAS-13B Thermal Weapon Sight.
Below: Mark 19 40 mm grenade ammunition locked and loaded during Operation Cold Steel II
U.S. Marine Corps Sgt. Robert W. Waterman, Dillan M. Ericksen and Cpl. Matthew T. Kidd with Battle Skills Training School, 2nd Marine Logistics Group, fire a Mark 19 40 mm grenade machine gun during a live-fire training exercise on Camp Lejeune, N.C., June 29, 2018. The Marines familiarized themselves with the operating procedures of the grenade machine guns to enhance their ability to train Marines and Sailors who attend BSTS.
I have never seen the Mk 19 being fired this way, but I see no reason why it shouldn’t work using it as a mortar for indirect fire. In terms of ballistics, it is a potato thrower in any case.
Pictures by U.S. Army Reserve photo, Spc. Brianna Saville/416th Theater Engineer Command and U.S. Marine Corps, Cpl. Sean M. Evans (just above).
Below is a video of the 40mm MK19 Mod 3 Automatic Grenade Launcher.
The MK19 MOD 3 40mm advanced grenade launcher is an air-cooled, blowback operated, belt-fed weapon. Highly portable within small soldier units, the weapon’s high lethality and broad versatility make it the prime choice of U.S. warfighter as an essential weapon in both offensive and defensive operations.
Firing M430A1 High Explosive Dual Purpose grenades, the MK19 provides lethal fire against a variety of targets, including lightly armored vehicles and dismounted infantry. It will penetrate 75mm rolled homogenous armor at a maximum range of 2,050 meters. Dismounted personnel within a radius of 15 meters from impact will be immobilized by blast and fragmentation.
FN Herstal also have a history of producing Mark 19s, and still make spare parts: https://fnamerica.com/products/grenade-launchers/fn-mk-19/