For anyone considering building an AR, please take some time to research and familiarize yourself with this chart put together by Rob S.:
http://spreadsheets.google.com/pub?key=pwswheghNQsEuEhjFwPrgTA&gid=5
Sadly, gun shops are typically very poor places to get information on ARs. Parts are not parts. If someone says something like "Brand X is just as good as Brand Y and costs half as much." you can safely stop listening to them. There are reasons why some items cost a lot. That doesn't mean to have to buy the most expensive stuff to get good stuff.
As people who appreciate the little (and sometimes big) differences that make RAT Cutlery knives better than other knives that cost less (and often knives that cost more), you should take some time to learn what makes some AR components better than others. Things like intense QC, HP and MP testing, better alloys, taper pins, properly sized gas ports, H buffers, and proper springs cost more but in the end make for a better product.
In my experience, the lower that you use make almost no difference as long as it is in spec (yes, I have had out of spec cheap lowers).
Quality lower parts kits are hard to find. It used to be that you could get LMT parts kits that were great. Colt kits are pretty much the best but will run about $200. G&R makes a great lower parts kit but you have to buy a lower from them to get one. Until recently we were stuck with CMT or RRA which both leave a bit to be desired. I had better luck with the CMT parts kits. Now Daniel Defense has filled the niche for an affordable LPK. They can be had for $70ish and it is a TOP QUALITY kit with every metal part in the kit made right there at DD. You can avoid all sorts of breakage and tolerance stacking issues by sticking with a good LPK.
The upper is probably the most important place for you to not skimp. Most of what makes an AR reliable happens here. The main things you want to look for is that your bolt carrier group is MP tested, shot peened, and properly staked. If a manufacturer takes to the time to do all that, their BCGs are good to go. Look for a barrel that is HP tested and MP tested. MP testing is of less value on barrels unless they HP tested first (CMMG used to MP test only which is kind of misleading, I am not sure if this is still the case). These things and others are explained in the chart I linked to above.
I will no longer consider barrels with twists slower than 1 in 8" (no more 1 in 9" barrels for me). The versatility of the 1 in 7" and 1 in 8" barrels is excellent. Both will handle everything from 55gr on up to heavier projectiles like 75gr and 77gr. Remember, it is not the weight that dictates what twist you need, but rather the overall length of the projectile. Heavier projectiles are generally longer projectiles so it is usually easier to refer to them by weight rather than talking in hundredths and thousandths of inches.
I think the least expensive way to get most of the features above, is to just buy a complete Daniel Defense rifle or upper (if you already have a lower built). They can be had quite inexpensively if you know where to shop.
I would also point out that due to some of the nuances of the direct impingement gas system, a thousand slow fire rounds from a target bench may not give you the whole picture in terms of reliability. I had Bushmasters and Rock Rivers that would shoot great from the bench or when I was piddling around on the range. Taking your carbine to a high volume training class will tell the whole story. When hot gas is being dumped quickly into the chamber and things start getting hot, things get harry and sometimes things break. Training is so valuable because it tells you if your rifle works, if your gear works, and most importantly, if you work.
I don't mean to step on the toes of those who own any of the brands that I have listed in a somewhat negative light. I will say that I can back up what I have written here with a fair amount of time and experience invested in this stuff. There are Bushmasters (or RRA, or Delton, or whatever) that serve their owners very well. But it is more of a crap shoot. There really is no such thing as a free lunch anymore.
I could go on and on about more features that you may want to look for but I fear I am already getting too long winded. If I get a few moments later, I may type up some off my experiences with optics (hint: there are reasons that I own 7 Aimpoints). The chart really is a good resource so I suggest starting there.
), all of my ARs (and AKs) wear Aimpoints except for one, which has an ACOG (TA11G).