There was a post on the board asking what the best upgrades for an AR are. Not an easy question, but I thought that I'd make a seperate post relating what I did to build an AR for a specific purpose, in case it's of interest to the rest of you.
The number one thing to keep in mind is that the AR-15 is an extremely versatile platform. Therefore, a question like "what is the best upgrade", is pretty tough to answer without knowing the way in which the rifle will be used. Here is an example of what I did with mine, intended for use as a varmint rifle:
Started with Colt Match Target HBAR, bought in 1999. Brand new in box, virgin rifle. This was a very specific decision. I know that Colt has a lot of detractors, but with the knowledge of what I was getting into (large pin sizes, etc.) I went with the Colt because I felt that I could minimize any quality and performance issues because of their reputation.
1. Swapped out original carry-handle upper for an Armalite flattop upper. This is the best thing you can do with a carry-handle rifle, because it is the single key that really unlocks the versatility of the AR-15 platform. Beware the bargain upper - this is the base that you are building the rest of the rifle off of, don't skimp!
2. Purchased a set of ARMS #22 rings with integrated QD/return-to-zero mount. I researched scope mounts for over a month before I made this decision. Bottom line, I got an extremely solid QD/return-to-zero mount out of the deal with minimum parts count.
3. Chose a Nikon Monarch 5.5-16x40 scope to mount in those ARMS rings. Choice of optics is nearly religious with a lot of people, but again after months of research I settled on this scope as being most appropriate for my situation.
4. Swapped out the military handguard for a simple (non-vented) DPMS free-float tube. There are a lot of crazy handguards out there, with lots of stuff hanging off of them, none of which made sense for a simple varmint rifle. That's not to say that they may not be a good choice for a differently-purposed rifle, but in this case simplicity was key. While I was at it, I drilled and tapped the tube for a QD sling stud.
4. While I was swapping out the handguard for the free-float tube, I lost the front sight and put a gas block with integrated picatinny rail in its place. The rail was just kind of a bonus as I don't intend to mount anything on it, the whole idea was to get the front sight out of the way.
5. Adding the free-float tube paved the way for a Pachmayr grip combo, that includes both a grip and a rubber sleeve that goes over the freefloat tube. The grip is comfortable in my smallish hands, and the forearm sleeve insulates me from barrel heat and adds a nice little beavertail.
6. Changed my stock trigger group for a Jewell trigger. This is another near-religious topic, but I knew I wanted a 2-stage trigger and the Jewell had more "pros" and less "cons" than any other trigger for my intended use. Some like the RRA (both with and without 3rd-party tuning), so that was another option. In the end, I ordered up the Jewell, hit the wear spots with a muslin wheel and some jeweler's rouge for a little extra love, and popped it in. The result is a trigger that I'm more than happy with. Perhaps I could have gotten there another way, but I'm satisfied with what I've got - significant considering how picky I am about triggers.
7. As a final touch, I added a Harris bipod. I chose the LM-S variant, which is the lightweight swivel with leg notches. The leg notches are much more convenient in my book than the continual adjustment. The swivel feature negates the need to set the legs up for different lengths on uneven ground, which put another nail in the coffin for the continual adjustment leg in my book. The Harris 'pods are available in several lenths, the 9"-13" allows for both benchrest and prone usage. If you plan to shoot mainly from a prone/sitting position, then the next size up would be more appropriate.
This is a long post I know, but I wanted to relate more of the decision making process than what I decided on. There are just so many ways to go with an AR, the main advice I have is to research the hell out of things and take your time. It took me 2 years to get the whole thing together, and I am going to have awhile before I've spent as much time shooting the thing as I did shopping :) In the end though, I've got a rifle that I am happy with and don't feel the need to try a flavor-of-the-month gadget just in case it's better than what I've got. The thing will shoot .25" groups at 100 yards (I actually think I was part lucky in that respect), and has been very reliable. The only thing I may do in the future would be to swap the barrel, but I will have to shoot out the stock HBAR barrel first.