Covert Ops
A system I haven't played any of, but the looks of it seem solid for an action-y game. It's feeling is more action hero than anything with rules for martial arts and gun-fu. Like in Ghost Ops, you're apart of a mercenary organization that accepts missions from all over the world. I would probably call it closer to MGS5 than R6S, though. You can make your own base, just like the legendary Big Boss!
Mechanics wise, it's 1d100, roll under. You have your base characteristics and then skills, which is just half the characteristic it aligns with plus any points you have in the skill. You can take multiple actions in a turn with each having a bigger penalty, so at higher-levels it's entirely possible for one person to kill a group of people with some good rolls. Currency is more abstract to control the power of the players and to gloss over awarding cash to them. Although, a supplement does add an actual money system along with a bunch of other optional rules. Another one adds cybernetics to give it a near-future feel.
Delta Green
Sort of a weird pick since it's about lovecraftian and occult stuff, but I feel like its rules work well for "realistic" combat. And doesn't get too insane like Ops and Tactics. Survivability will be questionable when doing with supernatural stuff, but if you're going the Spec Ops route then your PCs should be able to take down any group of armed individuals. Roll a 1d100 and get under your skill. A Green Beret will have his marksman skill probably at 80% starting out, so real easy stuff. Guns are handled in an interesting way with its "lethality" system. It's basically roll a 1d100 and get under its lethality rating. Let's say a M4 firing full auto at a person has a 15% lethality rating. If you roll 15 or below, that person is dead. If you roll above, you take the two numbers you roll (say, you rolled a 45, so you did 4+5 damage).
The only thing that might be questionable is how you lose sanity for getting shot out and killing people, but you can ignore that by making your agent experienced/Used to Violence. It reduces your maximum sanity, but lets you ignore any sanity rolls for being shot at or killing someone. Along with the system of supernatural that lingers in the background. I say throwing in a vampire or some zombies wouldn't be a bad thing, to be honest. Adds some spice. Hell, maybe even have them raid some cult and shoot some dudes. Good for SCP styled games, too.
Ghost Ops
The only one I really played a decent amount of. The biggest downside of it is using Fudge dice. On the bright side, fudge dice provides a nice foundation of "how likely should this player succeed", but can also have a flaw where there are certain rolls that a character would always succeed barring special circumstances. There are some special things added to it. You have 3 pools to get dice from, depending on the situation. One of those pools allows for BULLET TIME, where you tell your GM what you want to do, he sets a difficulty, and then you use as many points from that bullet time pool to try to pass it. Then there's one for helping with small boosts to rolls and another for surviving. From what I remember.
It's got a very R6S feel to it. You're operators from all across the world, getting your choices of class packages and weapons from the group and country you originate from. Be a sniper, machine gunner, demolitions expert, drone operator, whatever. Every mission has a payout and that payout can be used to improve/buy a safehouse, get some vehicles, get better gear, the works. You also get paid more if someone on your team dies or if it's harder than anticipated.
There's an expansion to play as "Agents" instead of Operators. Where operators kick down doors and shoot the bad guy, agents are more when the mission needs a more careful hand. Agents come from various agencies like the FBI or CIA and have different classes compared to Operators. They can even call in operators if they need them.
There's a few expansions, actually. So many that I forgot which to look in for the optional rule I want to use in the game. There's a second edition coming out anyway, so I'd wait for that.
Sector X
Well, not really modern day. It's set in, like, 2050, but still has a general modern feel to it. You play as Dolls and work for a PMC called Gryphon and Kruger from the gacha game Girls' Frontline. Pretty good for living out your fantasies of being a cute loli firing a machine gun since the dolls you play as can take any shape and size (as long as they're female!!!!!). Mechanically speaking, it's just Dark Heresy 2nd Edition. If that's a good thing or not is up to you. I think it's a fine system, even if it's pretty married to its setting. It's got entirely new gear, you craft your assigned gun when making your character and can further customize it with attachments, and it makes the Psyker system into electronic warfare although there's no daemon summoning, of course. You pretty much have to read DH2e to understand how to play Sector X.
It's all free, to include the supplements that introduce vehicle rules, HOCs (crew served weapons like mortars, automatic grenade launchers, and rocket launchers), scavenging rules, and ELID monster hunting. While it offers new talents, it's not that large of a list, so looking at DH2e's talents would be good. The single man behind it is still working on new content for it.
