The following links basically cover the mods to the trigger, and it only cost about $.50 cents for a 1/4 inch long 1/4X28 set screw. I have done many ARs' using this method, however I use $10 JP light springs, and polish the sear and hammer surfaces, with great results. The biggest issue after adjusting the take-up to your preference, is to make sure the timing on the disconnector is right. You may need to do some filing/grinding to the disconnector arm, that catches the hammer, or to the front bottom of the disconnector, to allow the arm to move more forward, to catch more of the hammer. Make sure you test the reset, buy holding the trigger back, charging the rifle to an empty chamber, and very slowly releasing the trigger to reset, to make sure it catches properly. Test the reset with a finger pushing the trigger forward, and another finger pushing backwards on the trigger, so that at the critical reset point, the trigger doesn't jump forward. This is a safety test to make sure the reset is not so close that the disconnector releases and the sear doesn't catch the hammer.
Your not trying to make this a "full auto" rifle, are you? Not likely it would go "full auto", but if the disconnector doesn't catch, or is not fitted correctly, one of three things will happen. If not fitted/sequenced correctly: #1. After the bolt carrier closes, you release the trigger to reset and the disconnector releases the hammer, but the sear doesn't catch the hammer, the rifle will fire. Or if the disconnector doesn't catch the hammer at all:#1. With very soft primers, it could go full auto as the hammer is moving forward with the bolt carrier group, and pushing the firing pin forward, which would be sticking out and contacting the primer, but not with the same force as the normal firing sequence. This is called a slamfire, but an out of battery slamfire could also happen, which could cause serious injury. #2. The more likely scenario is the bolt closes on the round, with the firing pin and hammer forward, but with no primer ignition. Then when you pull the trigger, nothing happens, as the hammer is already forward. You have a failure to fire. In either case, this is why you want to make sure the disconnector is doing its job. You have to use some brainpower to make sure you don't make the rifle dangerous, and thoroughly safety test by dry firing, before safety testing at the range.
You also will likely have to file/grind material off the rear of the trigger base, so the safety lever/selector will fit in. Make sure you test all functions of the trigger and reset, with the safety installed, as the safety will change the results. The best lubricant I have found for the sear face and hammer contact areas is a light film of anti-seize lube. I use Permatex brand, #133A, in a small toothpaste style tube, that you can find at most auto parts stores, for around $5. Works great on "pressure" lubrication points, like the sear face and hammer. Very similar, if not the same, as the expensive sear lubes, sold by the gun parts distributors.
And yes the grip screw hole is normally tapped all the way through, but it is easy to tell, just screw the set screw in to see if it goes all the way through. Joe Bobs looks like it doesn't have to be threaded through, because it uses a smaller screw through the center of the grip screw/bolt. I have had three ARs' that the threads went all the way through, and two that stopped just short of fully through. If it isn't tapped fully through, just buy a 1/4 by 28 tap, at a hardware store, and run the tap through. Clean the threads and use compressed air to blow out what remains. Both the set screw method and the Joe Bobs Trigger Adjuster will need a thread locker, to make sure the adjustment screw doesn't move.
Bob
Junk Yard Genius.com AR-15/M-16 Pages, page 1.
15 Minute Practical Trigger Job for the AR-15
Second link is for reducing spring pressure. I do not use this method, but instead purchase a $10 JP light AR spring set.
