First let me say I have little (just a very very few fights) “real” combat experience but lots and lots of hand to hand training. I have studied Judo and jujutsu for many years. I have also some limited experience with Hapkido, Karate, Kali, Jeet Kune Do, and most recently I have started training in Krav Maga. I have also been involved in “Run and Gun” handgun shooting. I am going to outline some things to think about when writing about fighting and shooting.
The enormous gulf of difference in expectation between an experienced fighter and inexperienced. This is most true in ground fighting. The inexperience almost always have the wrong reaction and make bad choices. Having taught Judo, jujutsu and coached wrestling for many years, I see it every time. A Neophyte on the ground gives up opportunities, and does things as simple as turning the wrong way almost without fail. Things someone with even a little experience doesn’t think about doing. If you are writing ground fighting scenes and one character has experience the other doesn’t, it should be short. I recommend watching a class with ground fighting and some beginners. Ask the instructors to show you what you want to know.