It is a well-known fact that a bullet placed just against or about 1/1000" off the lands yields the best accuracy. With the AR-15, you must have cartridges that fit inside the magazine for rapid-fire strings. This is not a requirement for the slow-fire prone phase, though, since you can single-feed rounds. This frees you to load heavier bullets (like the 80 grain Sierra match Hollow Point) into cartridges longer than the magazine, which means you can seat the bullet near or on the lands. However, getting the correct adjustment is difficult and is often a matter of trial and error. There are gauges on the market that help, but if you are cheap like me, you tend not to buy them. Here is a low-cost alternative for your AR that you can make in about 1/2 hour that gives you a fast start on establishing the heavy bullet seating length.
Step one is making a cartridge case gauge. De-prime, full-length size with small base dies, and trim to correct length a standard .223 case. Drill out the primer hole with a 1/8" drill bit. I drill a hole in the side of the case BEFORE I size it so I don't get the case mixed up with the ones I reload. Next, drill out the inside of the case neck with a sharp 7/32" bit. Then, polish the inside of the case neck with 400 grit sandpaper wrapped around a 3/16" rod until you can slide a jacketed bullet of desired weight and shape into the case mouth using finger pressure only. You don't want the bullet so loose that it slides by itself, just tight enough to use some finger pressure to move it. You will have to do this by trial and error. Cut off about a foot of stiff 16-gauge wire (fence wire is fine) and make a small hook on one end. I simply fold over the wire about 1/8" long and mash it together. If it will fit into the opened primer hole on the gauge case, then it's the right size.
Once you've made the gauge you can start measuring your chamber. With your AR-15 open and the bolt carrier removed, insert a bullet of the type you want to test into the cartridge case gauge. Leave the bullet a bit further out from the case neck than you expect it will need to be and carefully slide it into the chamber of the AR. Then, with a wooden dowel, push against the base of the case until it slides in tight. This should cause the bullet to contact the lands, pushing the bullet back into the case as you push forward with the dowel. You can also use the wire to push against the base of the bullet to make sure it is seated against the lands. Fish out the cartridge case with the wire hook inserted into the primer hole and measure the overall length with a dial caliper. Write down the measurement. Pull the bullet out of the gauge a slight bit and test the chamber length a few more times. If your bullet is too tight in the case mouth, it will stick in the lands and you will have to use a cleaning rod to pop it out. Smooth out the case mouth until that doesn't happen, but still puts sufficient pressure on the bullet so it doesn't slide out by itself. If you get the case neck too open, roll the case neck hard against the side of your table, which should tighten it a bit. Make sure that you get the correct reading after doing it at least three times. That should give you a good indication of the overall length for that particular bullet and that particular gun.
Now that you've taken measurements you can test the OAL. Make up a dummy round (no powder or primer) using a properly sized and trimmed case and the bullet you wish to use. Set the OAL length to that which you established with the gauge and, importantly, turn the outside of the case neck with a case neck trimmer or file it down a slight bit. You just want to shave off a bit of metal so the case neck is not too tight if the bullet gets stuck and you need to pound it back into the case. Smoke the bullet with a candle or match (or even a felt tip pen) and single load it into the AR. Let the bolt carrier release down on it the way you would in slow fire. Then, carefully eject the dummy round. Try to catch it or eject it into a soft towel so you don't get stray marks on the bullet. Make sure you don't leave the bullet in the barrel and that it will eject fine. If it won't eject, you can always tap the bullet into the case from the muzzle using a 3/16" brass rod if you have turned the outside of the case neck, but I like to avoid that if possible. A little moly on the bullet would help. Check to see if any lands marks are on the bullet. If you have problems ejecting or there are lands marks, reduce the OAL by 1/1000". Remark and recheck until you don't have any ejection problems and you don't have lands marks on the bullet. That should be a good OAL for that bullet in that rifle.
Note that because of the differences in the ogive shape of each type of bullet, you will need to go through this procedure for each different make/grain bullet that you want to use.