I have read things about DPMS and others being picky about feeding different types of ammo and that the same ammo fed great in their buddy's bushmaster etc.  

My question is what is the part of a bushmaster, for example, that allows it to feed better or be more reliable?  Or more precisely, what would you replace on a rifle that was picky to make it more reliable when feeding ammo?  

I am new to the group and am piecing together my first AR.  I am in the process of picking the upper / barrel assembly.  The pieces I have so far are all colt or bushmaster (with a stinger lower), mostly because I have found some good deals, but am excited about getting the upper together.

Thanks in advance for sharing your knowledge.  This site is great.

Bushmaster uses chrome lined barrels and chambers that are slicker for more reliable extraction.  Bushmaster also uses a 5.56 chamber which as I understand is microns larger than a .223 to accomidate micro manufacturing differences/variability in ammo.
Barrels such as the DPMS have tight chambers which result in slightly improved bench accuracy but may do so sacrificing some reliability.  So depending on what kind of shooting you want to do you can decide on what barrel you want.  Anyone please correct me if I am mistaking in anything.  

Well, that could be a NUMBER of reasons.

Overall I would say component quality (as a whole) reflect the rifles reliability. Cutting corners in manufacturing and skimping on quality controll are big factors. They are also big reasons why I would like to buy the best components possible for rifles that you might build.

Their are a number of smaller reasons related to individual part quality or worn out/dirty components.

*Could be the chamber. Shooting 5.56x45 in a 223 chamber may cause some finickyness.
Or it could be too tight. If the rifle is having extraction problems, the chamber may be a pretty    tight squeeze for the brass going in and going out. This can correct itself after a long rifle break in period. ARs are said to get smoother as they get older.

*Could be the extractor. The extractor may be worn out or the springs may be weak.

*Dirty chamber. Not good. This makes it tougher to extract brass.

(edit... I forgot about the chrome lining... )

Your upper is good stuff so far. The components you have chosen thus far reflect good quality on the part of both manufacturers. Build it up, and be SURE to place a good heart in the rifle.
Get a bolt that reflects what you want this rifle to be. A plinker? Get a junker bolt from oly or a kit manufacturer. A good rifle? Get a RRA bolt or Colt. A great rifle... Go LMT or Les Baer.

Right now, top of the line bolt IS LMT. It is state of the art as far as AR15 bolts go. More reliable design, better built, easier to clean, better extractor, ect.

Spend the dough on a nice bolt for your rifle. Years down the road it will show what superior quality controll and precise manufacturing coupled with a revolutionary design can do for your rifles life and reliability. I may come accross as a rich punk, but I am not... If I want a good car, rifle, fridge, shoe... I save and buy the better one.

You have collected great parts so far, dont skimp... the upper makes or breaks a rifle.

Thanks for the great replies.  I have a colt bolt assembly anxiously waiting for the rest of the assembly.