I am purchasing my first ar this week.  I am going to buy a colt 6920 from one of two local stores who always have them in stock. The one store stocks them new and sells used ones on consignment.  I know the first thing I want to do (after shooting it) is to start looking for a scope for the rifle.  My current favorite rifle is my remington pump action .30-06 with a 4-9x leupold scope.  The mounts for the scope on the remington raise the scope about 1.5" so you can use either the iron sights or the scope.  Is there anything similar to this available for the ar? I tried asearch and ddidn't get any results back.

There isn't a good way to do it. The optic would need a ridiculous height over bore to use irons through the rings. If you want both, I'd get flip up irons and a single piece quick disconnect mount for the optic.

Offsets are the only way I can think of.

In my opinion, the best way to do this is an ACOG (i prefer the compact 2x or 3x) mounted to an A2 carry handle.

I have a couple like that and it works really well.

Unless you go with a red dot like an Aimpoint it would be difficult, you could go with an offset mount or, if you want a fast in close sight picture, try practicing by sighting down the side of the scope base.
Some three gun guys have been doing it and at CQB ranges it seems to work, just tip the gun to the left slightly, like if you had offset sights.

I ended up purchasing a used 6721 with an aimpoint Pro and an mbus rear sight.  Keeping the stock front sight for now.  I will most likely put the red dot aside for a sbr build I'm planning and go with a leupold 3-9x with 45degree offset irons on this gun.  Wish there was a way to mount the scope on an offset and leave the irons on top where they belong.

BeowulfX

There are a few ways you could do this

1. Optic with quick disconnect mount and folding Back Up Iron Sights (BUIS)
2. You could run a Red Dot Sight (RDS) with Magnifier (most are 3x) on a flip mount and run fold down BUIS. Doing this you would then by cowitnessing the red dot, with magnifier out of the way.
3. Use a Optic with standard rings, and then use 45 degree offset BUIS

I'm going to go with option 1. Leupold mark ar 3-9x40mm with mildot reticle on a quick disconnect mount.  Saving the red dot for my sbr build next year (it came free on my used rifle).

For a magnifying optic? Probably not any good way. Some optics come with a rain/emergency sight on it OR you can get canted sights that mount the BUIS sideways.

For example, KAC has a side mounted BUIS for about 300 some bucks on Brownells: http://www.brownells.com/rifle-parts/sights/sight-sets/ar15-m16-45-degree-offset-folding-sight-sets-prod68387.aspx this way you don't have to remove the sights at all.

For ACOGs some model comes with BUIS on the top rails. Or like an Elcan comes with a BUIS on the top part that allows emergancy sights.

Now you can mount the scope on the charging handle with a rail that allows you to channel the mount to use the sights on the handle. Like this: http://www.brownells.com/optics-mounting/rings-mounts-amp-bases/rifle-bases/ar15-m16-carry-handle-mount-prod67333.aspx or like this: http://www.brownells.com/optics-mounting/rings-mounts-amp-bases/rifle-bases/ar-15-m4-carry-handle-mount-prod1572.aspx

Other folks here suggest using a quick detachment mount and that is good advice. Something like this: http://www.laruetactical.com/larue-tactical-spr-m4-scope-mount-qd-lt-104 will serve you well.

For most folks with a magnifying optic what they do is piggy back on it with a RMR or Docter optic that allows them to easily engage close ranges. Even with a 1x scope you're still at the mercy of the eye relief and with a RMR or Docter, it allows you to quickly engage targets up close without having to look into the optics.

MY OPINION is to not use any iron sight... of course I have BUIS on my setup but quite honestly I see it more of a aesthetic thing than something of function. I might one day at the range get bored and take off the optics and use the buis, but reality is at no point in the history of optics will you have to remove the optics and use the Iron Sights. I know a lot of folks think they might do but if you buy a good optics, you'll NEVER have to use the buis EVER unless the Hulk took a dump on your optics.

I've done the carry handle mounted optics before. It's so-so.

You need to make sure the eye relief is reasonably short, because you can't easily move the optic forward on a carry handle. You also end up with more of a "chin weld" instead of a "cheek weld" unless you get an optic with a short built-in mount or you're otherwise able to use super-low rings. I'm actually thinking about setting up a gun like that again.

If you're wearing a paintball mask (i.e., airsoft) carry handle mounts rock...except that airsoft guns usually have a 3-piece upper receiver that isn't able to put up with the weight of a carry handle + mount + rings + scope without wobbling...