I am in the planning stage of a new build. I already have a Mega lower and plan to order a Mega upper with forward assist, ejection port door and charging handle. I am leaning towards  a 20" stainless barrel with full float handguard. Also thinking chrome bolt and carrier. Will the chrome carrier and bolt make the action smoother and dependable enough to justify the cost? Who's barrel should I use and why? Keep cost within reason. I don't compete I just want a dependable accurate rifle. I want a 1:9 twist.
Any ideas would be appreciated.

Check out   http://www.model1sales.com/   it's a cool web site where you can get an idea of what it'll cost to build up a rifle.  Personally, I think you'd have been better off from a money stand point to get a stripped lower and then buy a complete parts kit.  You can check the prices and compare and see what I'm talking about at their web site.    Unless you're  a detacated varmint hunter I'd get an upper with a carry handle so you'll have sights, you never know when a scope will get bumped out of adjustment and having back up sights could be a life saver.   Also, depending on your tastes you could get a green furniture kit.  I think it looks good with a stainless bbl. and , lets face it, everyone and their grandma has a black stock so green is alittle different.    Check out their site, it's very cool.  Talk to ya' later.  TN.Frank

I'm not familear w/ the chromed carrier and bolt.

First of all, how much are you willing to spend?  You said with in "reason".  One person's reason maybe $1000.00 while another is $700.00.

There a lots of places to get parts.  Model 1 is one place, Rock River Arms is another.  M & A (IIRC) is another. Most places that sell parts sell good uppers.  If they didn't, they would eventually go out of business do to the bad repuation created.

Most uppers out there are 1:9 twist from what I see.  It seems to be "standard".

Don't waste your time (and money) on the chromed carrier and bolt, just make sure the inside of the carrier is chromed.  If you have money to burn, get the chromed assy, it's prettier, and easier to clean, but that's those are the only benefits AFAIK.  The effect of a chrome carrier against the aluminum receiver is a topic of much debate.  Even if it "made the action smoother", you would not notice it.  If you are on a budget, a chromemoly barrel is fine, but the chrome lined barrel will last longer in general.

Guess I should mentioned. I already have a Bushy M4. I have also built a 20" from a Mega Lower and an M&A kit. I want a flat top this time to use as more of a scoped target rifle. I am choosing the Mega upper and lower because I like them and want a stainless barrel for acuracy and ease of cleaning, plus I think it looks cool. Within reason meens I don't want a custom barrel costing $400 when one from say Oly. or DMPS for say $250 will do fine.

Ah, I see....

Well, since you are venturing to the territory of assembling an upper.....   I have not done that (yet?).

But you are right about barrel cost.  A barrel from X vs Y maker may not be that different.  My experiance w/ Oly is, not good.  I ordered a 6mm PPC barrel a while back and the ship date was pushed further and further back untill I just canceled the order.  Granted, a 223 HBAR in 1:9 20" is probably more common.  And you can order it "complete"...as in has the hardware to install (barrel extension IIRC).  And the gas hole is drilled too (I think).  Take a look at their site.

Then I think you answered your own question about the barrels.  I haven't heard anything bad about the Oly barrels.  However, if you are going for a target upper, in the long 20", you may want to consider a faster twist.  People who shoot target/competition with the nice long heavy SS barrels tend to shoot the heavier bullets (69-77g) and you need a faster twist for that.  I have been spying some nice 1:8 SS barrels that split the difference.

Mike,
    I don't plan on competing. I also want to be able to shoot 55 grain bullets. I am just looking for a gun for punching paper at the local range but want it to shoot well.

For 55 grain bullets, you can go 1:9 or even 1:12 twist. That allows you to optimize for 100 yards. Also, light varmint loads for hypervelocity.

Try this- build an upper off a cheap but good EE 20" barrel chopped down to 18". Now you got something "custom."

Otherwise, sounds like you want a RRA SS  20" 1:8 twist barrel. Out of the box, good to go.

OH! I got a Medesha forend which goes with the RRA very nicely, since it mounts on the standard barrel nut! Just remove the front sight (RRA pins are not tapered), and easy, easy easy from there. A bit on the expensive side, but super nice. Like, $100 range, and Scott is a real good guy.

Pete

Now that is good advice.  Cool man, reeeal coool! [8D]

Remember that if you are not going long range which the heavy bullets are needed for, you don't NEED a 20" barrel.  Longer does not equate to more accurate, just more velocity, as long as the barrel is long enough to stabilize the bullet.  If I were you, but still me, I would do exactly what StinkeyPete said.  Get a 20" 1:8 SS bbl, cut it down to 18".  It would be cool and different, long enough but not too long (I like short barrels), SS which is great for cut downs - no worry about the chrome peeling deal/debate, slow enough for 55g, fast enough for heavier bullets if I wanted to.  If you really are going to ONLY shoot 55g, then yeah, get the 1:9 twist.  Again, StinkeyPete really said it all, better than I could.

I'll echo the sentiment that you don't need anything longer than 16" to do what you want. If you get the right load, that 16" will be just as accurate, and lighter to boot.  M&A has standard contour match grade barrels for a lot less than $250...if you're not competing, why spend more?  You should be able to add chrome lining and still come in under $250 by a good margine.  If your adamant about SS, then check out Model 1...they use Shaw barrels, from what several people have heard, and they have a pretty good selection of barrels in SS.  

BTW, I've got 2 chrome lined barrels, and 2 non chromed.  I don't find the chrome ones that much easier to clean, a few less patches, but it's not that much of a chore to clean the non chrome ones.  

As for the chrome carrier/bolt assembly, save your money.  AR 15s have been running fine for years without them, you can put the saving towards the float tube.