As has been stated by others, experience is training. The ideal situation involves the highest level of simulation and then the actual experience which tends to sharpen the skill tremendously. I like to think about our special forces warriors when thinking about the ideal situation. When experience happens under life threatening circumstances, you have stress innoculation which prepares you even more the next time an event happens. You program the "mid-brain" for the next time.
What is interesting about experience is that by winning actual confrontations, you are more likely to continue winning. It is called the "Ace factor." A simple analogy for me is the phenom in which the same NFL quarterbacks tend to lead their teams to championship football games year after year. This year was no exception. These guys are Aces because they've been there and they stay cool under pressure (think Joe Montana). The same thing happens in gunfighting, whether on the ground or in the air.
As far as gangbangers who may have a lot of experience but no training, the continuous quest for survival coupled with their predatory nature, sharpens their abilities. Surviving conflict makes them stronger and harder to defeat. It is the law of nature: only the strongest survive.