A lot of shooters gain a little experience and realize that improving their AR trigger makes shooting much smoother. I’m not a pro, but I’ve done this a couple of times.
How to install a trigger in your AR.
https://youtu.be/QTFeV6hiT4I
In this video the trigger is from Jard Inc and lower from Diamondback Firearms.
http://www.jardinc.com/pagear/