Alright, so again, I didn’t just randomly google these topics. I was in Iraq at the same time. I was a marine. I am deeply intimately familiar with the system in question. I currently synthesize high explosives. I have participated in ballistics research and high explosives effects research in Aberdeen proving grounds.

I was there, and used the weapon system in question.

That out of the way, here is some nuance.

Marines taking indirect fire were authorized at the time to use indirect fire weapons to suppress that indirect fire. By definition, indirect fire lands on a target you cannot observe. When an infantry rifle squad employs indirect fire from say, a M203, it is because you cannot hit the target with direct fire of a rifle or cannon.

A mk19 is simply a larger version, but the rounds almost universally issued as HEDP. The majority of their utility is in being light armor penetrating because they are constructed with an inverted cone that is base detonated. It sends the majority of its energy into the direct front of the impact in a focused plasma from the explosive detonation in the projectile. It is surprisingly ineffective as an area fragmentation weapon, even when labeled as dual purpose. I watched them get fired at attacking insurgents where the grenades detonated right next to them along a wall and do no damage to anything but the small hole in the wall of that explosive jet. On multiple occasions.

As for employment, we used indirect fire, regularly, in theater against incoming indirect fire. This was done in often, urban environments and cities. Most of all the fighting in the country after the initial invasion weeks occurred inside those cities, because no real point in fighting in open desert for nothing.

So to be very clear, mk19s were employed OFTEN in operations in urban areas, against indirect fires, as indirect fire suppression.

Further still, it is the literal smallest indirect fire weapon option to exist in the arsenal, so you could not be more judicious to respond to incoming fire than the use of a 40mm grenade.

I personally watched firefights where we used them to similar effect though not anywhere near as much advanced planning was used as he described in that Reddit post. Using the marine corps published calculations for trajectories, mapping out impact areas in advance to ensure accuracy to the limits you can within a remote FOB, is the work mortar men do to ensure accurate fire returned.

So if every single incident of returning indirect fire is a war crime, then there are a hell of a lot more war criminals in the military that need prosecution.

To be very clear, we never should have been there fighting in cities in unjust war or inversion, but it is incredibly clever ingenuity that chose the minimal explicit yield possible, with lots of effort specifically to avoid collateral damage when used. The pre-sighting described and calculating trajectories is not the work you spend weeks on if you intend to harm the wrong person.

You can believe no indirect fire weapons should ever be used in cities, and that is a fine enough opinion. You would be saying that in he face of everyday single conflict in the history of warfare in the last 100 years though and all people involved in indirect fire in places not entirely around military occupants as war criminals. Done enough opinion, but that is a vastly different interpretation that what is currently followed as a war crime in ANY modern conflict.