Below is the red AR 2 stage short trigger I installed as a drop in unit. It was not difficult to do once I pulled the old trigger and the safe/fire switch. Also I had to remove the hand grip with an Allen wrench to reveal the spring and tension for the safe/fire switch for removal. All to get the trigger and its base out of the lower unit.
The AR Targa Image is from the Timney web site. http://www.timneytriggers.com/shop/AR-Targa-2-Stage-Short-gger-PTri77.aspx
Some folks have already made a You Tube video of a Timney Install on the AR-15 and it is a good one. I followed it easily. The great part is that if you can work with basic tools it is a drop in solution. No muss no fuss.
It took me 45 minutes to do the removal of the old trigger and install of the Timney with a You Tube video at my side to follow. Since my lower unit is metal it did not have the little loose lock screws as the video shows.
The trigger allows in the first stage to pick up slack at 2lbs so you know you are pulling, then firms up and releases the hammer at an additional 2 pounds. I was not able to use a Trigger Pull gauge to determine the 2 pounds to break the sear it but is supposed to pull 2 pounds in the first stage as far as I can tell and then another 2 pounds to break the sear. It is smooth and breaks crisp. My groups have tightened with the tightest less than 3/4 inch at 100 yards by my 3 shot groups today 3/20/16 and my worst group at 1.5 inches.
Now there is no excuse for the trigger to provide poor groups and can work on powder, bullet choice, Cartridge Overall Length (COL), Scope and Cheek weld to enhance the groups even further. At least that is the plan.