I am a newbie to the list (and AR's).  I have a stock Colt AR-15 competition HBAR flat-top that I want to use for target shooting and as a back-up gun on a prairie dog hunt.  I have about $400 to spend on the gun to make it more accurate.  Any suggestions?  Thanks!

Get a RAS FF to free float the barrel (and allow for attachement of a bipod w/o moving the point of impact).

That assuming your rifle isn't already free floated (IIRC the Colts with the 24" barrels come free floated).

Get some USGI magazines, some good m193 or m855 ammo, and some hunting ammo.  Take it to an outdoor range and shoot it and see if the accuracy is adaquate (most ARs are pretty accurate out of the box).  A free float tube with bipod is also good.  

In my view, the single greatest improvement you can make is installing a better trigger.

Go to this page and check it out. It is the Highpower riflesmiths site. Frank White who is one of the undisputed gurus of accuracy with AR-15's tells you what he thinks is the best things you can do to improve your accuracy and he puts them in order. In order they are free float the barrel, rear site upgrade, front site upgrade, weight and balance, trigger replacement and barrel replacement. Here is the page. http://www.highpowerrifle.com/Accuracy%20services.htm

If you don't have a nice scope and ring set. I would look into that first.

Although the FF RAS is a VERY nice tube. For target and prairie dogs, I think something without rails will suit you better and be less expensive. Then you will have more money for other things.[:)] (sorry Forest, It's my Scot's blood).

My personal favorites for varmint guns.

I don't like any of the solid aluminum ones.

Hiperform Carbon Fiber FF tube. Light and strong, doesn't heat up. ~$100.00. You will have to drill your own hole to attach a bipod.

Armalite fiberglass FF tube. ~$90.00. Heavier than hiperform, but very strong, and indexable and has an attachment for a bipod already.

So. I would get a FF tube for $100.00 and a harris bipod ~$60.00.

Then you'd have enough left over for a match grade trigger and more.