New RRA 20" A2 is on the way. Is there a procedure/recommended way to break in a new gun?

What ammo should I use? (Yeah, yeah, preferably .223....)

Mugzilla

Type of barrel affects type of break in.  Assuming your barrel is chrome lined, you simply need to take gun to the range and fire few hundred rounds, then clean it.  You can lube moving parts on the gun before taking it to the range.  I recommend Bushmaster's tech support page at www.bushmaster.com/faqs/.  Just don't be disappointed when you come back from the range to find out you got scratches and dings on the rifle.  Thats normal :)

Chrome lined or not, you just invested a couple hundred, why not play it safe?  

Clean the barrel as soon as you get it.  Nothing major, just to get any leftover machining crap out.  After one shot, clean it.  Do that for the first 3 shots, then I'd suggest blasting 20 and cleaning then go nuts.

Better safe than sorry!

1.  Load up a Beta-C

2.  Switch the selector to "auto"

3.  empty the mag.

4.  Repeat.

Over simplified explaination:
Break in consists of firing approx. 200 rounds with a strict cleaning regiment after each few rounds.

Instead, try DAVID TUBBS FINAL FINISH ammunition.
It saves time , money and frustration.
I've used FINAL FINISH to break in three match rifles...great results.
Follow the directions!
www.arizonaammunition.com/Coated%20Bullets.htm

Armilite has a long break in proceedure on their web page, and recommend moly coated bullets. They recommend cleaning after each round fired for the first 20, then after each 3 for the next twenty, then clean after each twenty after that.
There have been posts that have a difference of opinion on break in however, and I wished I had read all those prior to doing this with my 2001 legp.
Basically their are those that believe this is all hog wash and is just making you wear out your barrel faster. Your extra cleaning just takes a couple hundred rounds of life out of your brand new barrel. It makes sense if you read the whole post/artical on it.

I fanatically follow ArmaLite's procedure with any of my rifles, rimfire or CF.

In fact, I spent 2hrs, just this past weekend, breaking in a new M15 20"SS Target barrel and fired a whole 20rnds during that time.

I DO NOT believe in using brushes, at all!
Just patches and weed-wacker line.

That barrel costa nice chunk of change and I'm gonna get my money's worth from it.

Boring? Hell yeah! BUT, ya bring a few other rifles, with ya, to break up the monotony.