This is a tough question ? I need a rifle to fill a niche. I have a Nat. Match M1A for 300 to 600 yd. work , a .300 Ultra 700 PSS for the 600 to 1000 yd. work. A .50 BMG AMAC for the 1000 to ???? work. I need a go to gun for the close in work , I cannot decide upon a 16 or 20 inch AR with all the toys , C-More sights , etc... Please help..

16" with a dot. good for 0-200.  i would go bushy flat top.  Eotech and ARMS M40
or Aimpoint forward mounted on an ARMS super swan.

DITTO

Ditto

16" all the way.  20" sucks for CQB.

Oh Boy!
You came to the right place!
You should get about 10,000 replies to this.
14.5 or 16 inch M4 style Colt, Bushmaster, RRA.
Site: Trijicon, EOTech, Aimpoint.

Welcome to the site!

P.S. Lumpy, I miss Taxi Driver!

14.5" for a preban 1/7 twist barrel.

16" for a post ban or 1/9 twist barrel.

You gain very little going to a 16" barrel in terms of accuracy or fragmentation range if you go with 75 or 77 grain ammo.  The shorter barrel makes all the difference in the world as to whether or not you can keep your muzzle up as you slice the pie in a tight hallway.  The better fragemtation at 150-200 yards in the 16" is not very likely to mean much.  Having your muzzle pointed down as you round a corner and have a gun pointed at you instead of up on target because the rifle was too long can mean a lot.  The 20" will suck donkey balls for CQB.

Oh I also see your new.  Go to [url]ammo-oracle.com[/url] and read up.  You want an Aimpoint or Eotech for your optic.  Id pass on a Cmore.  If you want magnification get a Trijicon TA31 or TA11.  You will want a Surefire light too.  M500 if you dont want a rialed foreend or a Millenium if you do and a M900 if you want a foregrip and light.