Flat top, 20". I could use some ideas. Does anyone make one with a bullet drop compensator. Got any pics?

Umm, what are you going to use it for? Deer hunting in the Maine woods with shots of 45m? Shooting paper at 100m, hunting wulverines in Alaska with Napoleon?
What are your base needs?

Scouts Out.

leupolds custom shop will do custom target nobs for there line of scopes with a bullet drop compensator.

+1 on giving us some ideas of what you plan on doing with this rifle

Agree that  need more information, however, the question of what optics system are "general" answers to an unknown "question" in of it's self is a good guestion:

Aimpoint setup to cowitness with iron sights and possible with a X3 maginifer that comes off easily.  The Aimpoint (or Eotech) are the best for this side of 35 yards (really the only thing we see work well for optics in Subgun Matches).  A "dot" type sight works ok out to several hundred yards, and with the X3 maginifier unit added one has a nice scope system for past 300 yards on reasonable sized targets (shooting tin cans at any distance past a 100 yards, still like the X10 scope).  

1.x to 5 scopes give a good field for view at closer distance and the standard "hunting" VarX-III Leupolds in 1.5x5 would work as well on an AR as a low cost option (same scope works well on  hunting rifles/shotguns in the Midwest).  For an AR might wish to add illumination or look at one of the "more tatical" low power scopes that you can use easiler with both eyes (all can be obtained for a price - S&B @ $2K+ ;-).

ACOG X3.5 setup for .308 probably will be close for BDC.  Nice general purpose lower power scope.  That with the BAC one can still use at 0 to 7 yards nicely.  Advantages of the ACOG are a very sturdy scope, that has clear optics and no batteries, knobs etc.  - nice KISS system.  At a distance the system is not as nice as the 10 power scope (can not make up for maginification) and close up the Aimpoint is a better system - however of all the systems only the ACOG can you pick up and shoot any time, any place with out doing anything (except maybe taking the scope cover off).  For a general issue miliary scope, works great, for a general usage scope is good, for any one applicaiton it is almost always the 2nd choice (but this is it's strong pont).

3.5 to 10 Leupold Mil Dot for "unknown usages" at mostly 50 yards plus.  The low end power is not bad for off hand shooting (would like less power on close moving targets) an ten power is enough for nearly any reasonable usage of an AR system.  Once you figure out how many "dots" to hold over as the range increases the "dots" work as your BDC (i.e. one dot at 300 yrds works on a couple of my rifles that have zered at 100 yards for most shooting of "tiny" targets).  If you are not shooting inside, nor shooting action targets this side of 50 yards nor shooting running game in the brush - then it is very hard to beat a good variable power scope with some type of ranging system (be it mil dot or a something more specialized).

Regards
John