The things I usually add on or change on my AR's is:
1. The stock
2. The grip
Its hard to fall in love with shooting a rifle if you arent comfortable shooting it. I usually try "upgrade" the components that play most in the ergonomics and shooting experience.
Well since everyone is giving there favorite setup, this is mine. I like for all my rifles that are used for self defense to have a food quality light on it. No pressure pad, just a momentary on/off. Also a good set of iron sights. Red dots and holographic is cool but just one more thing to break. Finally I will adjust the ergonomic of the rifle is it doesn't fit me well. That's it fellows
