First off, I’m sorry this took so long to write up, this is a much deeper topic, and there will be some full articles on the subject coming in the near future.
But, to your question, the short answer is; not much, really. Fights at close range are very short, and will involve characters firing as quickly as they can at one another.
Some of the same assumptions also hold true, characters who have training and experience will win out over characters that don’t know what they’re doing.
The biggest difference is with guns, there is no playing nice. Any character that’s injured from getting hit will be seriously injured. Healing from a gunshot wound will probably involve months of recovery.
As with other weapons, guns are unique to one another. A character that’s used to using a USP .45 will be at a serious disadvantage if you hand them an M1911. Some of the basic theory and practice caries over, but the way you operate one is different from the other.
Bullets will penetrate light cover. If you’ve played a lot of recent military themed shooters, this should be a familiar concept, but games tend to undercut how severe this can be. If your character is opening fire with a handgun, there’s a real risk the bullets will blow through walls, cars, and whatever else, and hit someone they didn’t intend to.
In most residential or business settings, you won’t find cover thick enough to stop a handgun round, meaning the whole “take cover behind that couch/upended table/car door/lawn chair” tactic doesn’t actually work. Throwing a conference table on its side may look cool, but it won’t save your characters from getting perforated.
Military combat is a completely different animal. It focuses on long range fire, suppressing a target (keeping them from moving or firing back), while other squad members move in to eliminate them.
This tactic makes its way back into gunfights involving trained characters. In a firefight, their primary goal should be getting out of sight, and moving around to the side or behind their attackers.