Mourn is a retro fps style tabletop rpg, inspired by games like Quake and Dusk---although it feels a bit like Halo as well, and the book makes it clear that you should use the system to run the old-school game of your choice.
Mourn's PDF is 17 pages, with a bold and stylized layout that fits the genre. The text is a little small, but everything is cleanly arranged.
Lore-wise, you play as partisans fighting to reclaim a ravaged world and repel its invaders. There isn't much more than this, but with Mourn's breathless pace, you don't really need more.
Mechanics-wise, Mourn has a strange feel to it. Actions are resolved through a coin flip, which you can gain advantage and disadvantage on. Damage and health are big chunky numbers. Stats are upgradeable and can be refilled through in-game pickups. PCs that get knocked out can revive instantly if they can land a hit on a nearby foe, and can be recovered from resurrection altars otherwise---although doing this takes a chunk out of your stats, so "perfect" play involves never getting KO'd at all.
There's simple ammunition tracking, and melee attacks in case you run out, but killing foes recovers ammo so you'll usually have your range tools available. There's also a crit system that kicks in when you hit successfully several times in a row, and an execution system that gives you a huge health and armor recovery if you were near an enemy when you killed it, so you don't have to constantly break off from the game loop to recover.
Movement and line of sight are tracked, and are probably the most tactical part of the game, but movement is an upgradable stat and this means you can speed across maps. As a result the GM is sort of incentivized to build environments with lots of barriers, verticality, and obstacles---much like a classic fps.
Not using a battlemap is *possible*, but the game really feels like it wants one, and the core mechanics feel a bit flat without it.
Similarly, the game engine doesn't feel like it expects characters to do a lot of sitting around talking, so it might be best to keep most of your roleplaying to little vignettes, or as bits of dialog while bouncing about the map smashing enemies.
Character creation does feel good, though, and selecting your weapons and starting kit gives you a nice range of tactical options. You can essentially move sliders between high damage, high ammo, long range, and easy to use and get weapons that fit your playstyle, and there are also portable cover pieces, self buffs, and melee tools to select from too.
You have stats for use outside of combat, and they progress as you use them and fail, but they progress super quickly to the point where they cannot fail, so you might end up making some modifications here.
For GMs, there's a full bestiary and advice given throughout on how to run and mod the game. The system is also super easy to learn, and you can probably pick it up in twenty minutes.
Overall, MOURN is a super quick, kinda boardgamey adaptation of classic fps to a tabletop environment, and for that alone it's worth your time. But if you like fast, combat-focused tabletop games in a freewheeling beer-and-pizza style, and if you like to build little fps environments out of miniatures and terrain, this should especially be on your to-read list. You may end up making a few modifications to customize the game to your tastes, but MOURN both supports and expects that, and its engine is super easy to mod.
Minor Issues:
-Pages are numbered in the index, but not on the page itself. This isn't a huge issue in digital as the PDF tracks page numbers, but could be a problem if converted to print.
-Page 5, "thousands of species of conquered species" species twice
-Page 6, "land a successful attack with their weapon with all of their actions" should this be something like "they refill their actions and resurrect at 5 hp if they land a successful attack"?
-Page 6, getting max armor from a pickup and recovering half max armor on an execution feels like it has massive ramping potential. The GM can control this by simply not letting players get armor pickups, but is this intended?
-Page 7, it isn't clear if Executions round up. If you Execute someone with a MOURNER, do you get back its one bullet?
-Page 10, there doesn't seem to be a reason to take the revolver over the heavy pistol. Dual wielded revolvers is the same damage, range, and limitations as a heavy pistol, and a heavy pistol can be further dual wielded for even more damage plus disadvantage.
-Page 11, the harpoon gun seems to just be a longer range rocket launcher? Less damage, more ammo, or a tethering effect could differentiate it.
-Page 12, "give you an added a chance" no a
-Page 12, stats increasing by one point every time you fail with them seems like you're going to be getting guaranteed successes after one or two failures. x3 advantage on a coin flip all the time means you're pretty much never going to fail. Maybe stat points aren't permanent, but are instead spent to reflip the coin?