Get In Your Character's Head
To understand how to write a modern combat scene, you have to understand modern combat.
It is not impossible for someone who has never been in combat to describe it well, but it will certainly be more challenging.
First of all, you must get into the head of your character and STAY THERE. If you describe action from outside the protagonist's subjective consciousness, the action will rapidly become just a boring list of stuff that happens, and will drag down your story. Think about movies and modern TV shows: when there is action, they shake the camera, they show only little parts of what is going on, they narrow the field of view on purpose, they leave things unclear instead of describing the whole scene from a "God's eye perspective" if you will. This resonates with us human beings because of the way we physiologically react to those kinds of situations.
When you are in combat, your body takes a MASSIVE shot of adrenaline. It is equivalent of being high on crack cocaine. This chemical is produced by your body naturally and floods your brain and your major muscle groups, causing physiological changes. Your vision LITERALLY narrows, as if you are looking through a keyhole in a door. Your eyes can focus in on ONE thing at a time. Read: TARGET. Your brain goes into "Kill it, run from it, or F** it" mode. Literally, it is that simple. Your major muscle groups are radically stronger than normal and you feel like you can literally fly. Your minor muscle groups are starved of blood and you LOSE manual dexterity. If you have to interact with a computer keyboard in this condition, you will be frustrated, but if you have to interact with a club which needs to hit the head of a cave bear, you will be VERY pleased with the result. This is where those stories come from about people who reach out without thinking and lift the back end of their car off their husband as it starts to roll down the driveway.
So you are physiologically the equivalent of a guy so hopped up on crack that he can't feel pain. You won't feel any pain either until the action actually ends. You are on a high. Strangely enough, while this is happening, you become incredibly logical, like a Vulcan. This is because things are happening so fast that you have no time to emotionally process any of it. You just observe, see what happened, and put it away in a box for later. This is how you end up with things like some of the symptoms of PTSD where things that had never been emotionally worked through years previous are suddenly resurfacing and you have to open up that baggage that you had just compartmentalized at the time because there was no time to deal with it. While you are in "combat mode" you have NO TIME to emotionally process what is going on, you feel like a fly: moving at super speed, the whole world slowed down, things that need to be reacted to neatly put away and inaccessible, thinking in terms of pure logic: "How can I kill this guy before he kills me? What kinds of objects can I use in what way? How can I use my environment to my advantage". It's like all your brain's processing power goes into physical things, objects, physics, the art of the possible with WHATEVER happens to be lying around.
So you don't see a big picture, you see a pinpoint, and there is nothing but process potential threats and deal with them one at a time, no time for emotions. Don't say things like "their training took over". That's a cliche and there is literally no time to think about how much of what is going on in combat is from training and how much is imagination (Hmm, I bet I could use that pipe over there!). Another thing: your ears kind of shut down and you lose aural sensitivity. In modern combat, there are a LOT of VERY LOUD MACHINES like fully automatic weapons. stand next to someone cranking the throttle on a Harley Davidson and you will get the idea. Combined with your brain sort of "turning off" your ears, you tend to just not bother with them, your brain spends all it's neurons on the eyeballs. Humans are visual hunters, after all. People in combat tend to SCREAM AT ONE ANOTHER, even when they probably don't need to. They all act like they are coming out of a rock concert with ringing ears, yelling to be heard. Of course, in the moment, there is no need for talking (or screaming): there is only move, act, observe, calculate, kill.
You can probably get some sense of what it feels like for your character to be in combat if you have played in a sport team in school. Maybe you have been tackled in Football, maybe you had to sprint the moment the gun went off in track. There is a similar feeling in sports, though it is much more pronounced and a much higher scale in a real life or death situation. You become very much in the moment. Try to write the scene from the perspective of your protagonist and try to get into their head. Write subjectively, only what that character sees. Don't bother trying to explain everything. Sometimes there are flashes of non-sequiturs in combat: you see something that just absolutely makes no sense given your limited understanding of what all is going on. You see something very random happening in some room as you pass by for example, and never get a good explanation of what that was.
Time is also distorted. The moment combat begins, your internal clock is going a million miles an hour and time seems to slow down. You feel like you can count the beats of a fly's wings. There is a strange sense of peace, calmness, just before the explosion of activity as your body and brain accept that, yes, this is going to be a 100% bona-fide fight or flight deal in the next few seconds.
As far as writing style: stay curt, short, precise. Move fast, don't explain. Tell us what the character is thinking, but reflect that the character's brain is working kind of like a crazy addict at the moment. Not tripping, but hyper focused, very logical, and totally emotionless. Just state things that happen. Does a human body get blown into a million droplets of liquid by some kind of high tech weapon? Ok, the protagonist just goes with it. They will process the "heavy" stuff; what that means, later. They just note the stomach-punch of the "whump" sound made by the sonic weapon and the way their eyeballs feel like they jiggled in their sockets when it went off. Describe how a wall has been blasted to dust and the character has concrete grit between their teeth. I seem to recall concrete grit between my teeth just about every time we were in combat... You notice weird little details like that.
If you want a good model, read Hemingway. He was extremely concise to the point of almost seeming impressionistic, but he gives little flashes, little still pictures through the course of the action, which mimics the way your brain processes information in that state.
Finally I will leave you with one thing I learned about action scenes and action stories: Action in itself, is ONLY interesting inasmuch as it effects a character that the readers are invested or interested in. Readers don't care what happened, they care how Private Danny felt about what happened. ALWAYS DESCRIBE HOW THINGS EFFECT THE CHARACTERS right through the action scene, even if it comes in the form of: "He knew he would have to process the impact of XYZ's death later, there was just no time now to feel what he knew he felt". The empathy of the reader is the single, most important thing, not a systematic description of how a battle went down.
One last tip: reduce the cognitive load on the reader as much as possible in action scenes. Use small, simple words instead of big ones. Anything to keep the reader moving along rapidly. It gives the reader the feeling of things going fast, which helps with the illusion of "experiencing" combat.
