I thought ya'll might be interested in a cheap, reliable 4# trigger for your AR. Right up front, I'll conceed, the following proceedure isn't suitable for duty/military rifles. It's obviously not suitable for select fire triggers. It is, however, fine for competition, hunting and range rifles. I make no claims about being a gunsmith. What I am is a competition shooter with a good understanding of AR trigger functioning/dynamics.
IF YOU'RE NOT MECHANICALLY/TECHNICALLY INCLINED, FIND A GUNSMITH TO INSTALL AN AFTERMARKET TRIGGER FOR YOU. For the most part, the AR trigger group is a simple, straight forward arraingement that lends itself well to competent mods. Proceed at your own risk though...
Stock AR triggers are meant to be absolutely reliable under any condition imaginable. They are intended to pop hard, milspec primers after landing on the beach or under arctic conditions. They are also meant to be lawyer proof when some idiot AD's thru piss poor gun handling.
Most of us practice good gunhandling skills and would prefer a clean breaking, light trigger to extreme conditions reliablity. It suits our uses MUCH better. The following is an inexpensive alternative to expensive aftermarket triggers and offers performance almost on par with many of them.
This proceedure is a hand fitting operation that requires assembly/disassembly several times during the process. It's not difficult, but takes some time and patience. The results will be worth the effort!
Let's start with the hammer. Since a light trigger requires the use of a lighter hammer spring, the hammer itself needs to be lightened to maintain ignition reliability. Reliable ignition is a function of hammer SPEED, not mass, with the light spring installed.
I recommend JP hammer/trigger springs. The pics following show the modified parts on the left. I photoed them with the JP yellow springs installed for clarity. Hammers can be modifyied using a Dremel tool and a carbide cut off wheel. Cold blue as necessary.
Modified "speed" hammer:
Next, a 1/4x28 set screw is installed into the grip screw hole to push up on the trigger and eliminate the "pretravel" problem inherent in a stock trigger. Stock units have HUGE sear engagement and actually cam the hammer back considerably on firing. This is what we aim to remove with the set screw. When we get everything "just right" the set screw will be removed and reinstalled with blue LocTite. It must be chemically clean for this step.
Set Screw installation:
When the set screw is initially installed, the rear of the trigger will have to be ground to allow clearence between it and the safety. When properly set up, a modified rifle will have a "hard" safety. This doesn't mean that it's hard to operate, but rather that there's very little clearence between the safety and the trigger when the hammer is cocked and the safety is "on". Remember, we removed the excessive sear engagement with the set screw. Grind the rear of the trigger carefully. Reinstall it often to check for proper fit.
IF YOU GRIND TO MUCH, THE SAFETY WILL NOT FUNCTION PROPERLY AND THE TRIGGER WILL BE UNSAFE! The only cure for an overground trigger is a new part. Be careful. Polish the hammer and trigger engagement surfaces with Flitz compound. The idea here is a smooth finish, not to change any angles or dimensions. The surface hardness of the parts is only a few thousandths of an inch thick. If you go thru the hardened part, the trigger job will not hold and the rifle will begin to "double" as the parts wear excessively. For this reason, don't use anything except a polishing wheel and Flitz or stones for working the engagement surfaces.
Modified trigger:
Sometimes, when the sear engagement is reduced to reasonable levels, the disconnector won't release the hammer after a shot is fired and the trigger is released.
Some trigger jobs require modifying the disconnector "nose" to allow adequate clearance. This is done by carefully removing material until the disconnector will release the hammer properly. Be sure to keep the angles of the nose perpendicular as shown below.
Modified disconnector:
Another veiw of the disconnect mods:
When the modified trigger is installed, LocTite the set screw in place. Allow 24 hrs for the LocTite to cure before firing the rifle.
With a cocked hammer (unloaded rifle) and the safety "on", the trigger should be pulled to see if the hammer falls. Obviously, it should not. Sear engagement in the above proceedure is a common sense judgement. The trigger should break clean but not be a "hair" trigger. The hammer should not fall when the unloaded rifle has the muzzle or stock bumped on the floor (wood block). The resulting trigger from the photos isn't quite in the league with my full JP trigger in another rifle, but it's a HUGE improvement on the stock unit. It breaks clean at 4 lbs and has been totally reliable.
ETA an link to a downloadable pdf version Punani was kind enough to provide. Punani's pdf version