One beautiful thing about the AR15' s is the modularity of parts
and the rails that allow you to quickly reconfigure the rifle, the
Barbie doll for men, with enough accessories to last a lifetime!
The entire AR-15 can be disassembled in just a few minutes and
re-configured, at-will.
Also see my notes on
Mounting a rifle sight on
an AR-15 or M4 Here is the quick-release rear iron
sight from an AR15:

Co-Witnessing a AR15 sight
Synchronizing multiple sight types is called "co-witnessing", and it's
great if you want to combine a red-dot with a telescopic sight or a red
dot with an iron sight.
Because the front iron sight does not flip-down the front sight becomes
a distraction. I mounted a red dot sight on my AR-15 and
discovered that the iron front sign is quite annoying:

You must ignore the front sight on the older model
AR-15 rifles unless you “co-witness” the iron sight with the red dot.
Co-witnessing is also done with telescopic sights
and red dot, but it’s expensive and time consuming.
With co-witnessing, the iron and red-dot become
synchronized together:

Another approach: AR15 sight stacking
AR's are flexible and you can get carried away when adding
accessories:

Here is an experiment I tried on a .22 and my .223 AR-15, stacking a
telescopic sight on top of the existing iron sights.

On the .22 caliber rifle (with low recoil), this stacking worked OK,
allowing me to use both sights, but I had to take my cheek off of the
stock when using the scope.

Iron peep stacked under a telescopic sight

When I tried it on my .223 AR-15, the height difference was too much to
effectively use the scope.
So beware, even thought the AR-15
assault rifle has modular parts, sometimes you can combine them into
less effective components.
I solved my problem with a quick-release rail mount that allow you to
remove a red-dot of telescopic sight without loss of zero.
This make it only take a few seconds to change sight types, so I can use
all three sight types, depending on the distance of the enemy.
Note below that the stacking is not necessary since the front sight
quickly unscrews:

.22 cal AR15: low recoil, stacking is OK for sighting telescope
Another option if you have an AR15 or M4 with a fixed front sight is to
cut-off the front sight (about $100) but a faster solution is to use a
cheek riser.
Using a cheek riser also has the added benefit of preserving the
original military configuration for long-distance iron sight
competitions.

The Bell and Carlson
cheek extender
Using a cheek riser means that you don't have to mess with co-witnessing
or cutting-off the annoying front sight.
Cheek weld and high sights
Another option for high stacked sights is an extended
stock butt to allow for proper cheek weld.
Here is a process for installing an elevated butt
for high telescopic sights.

For the older A2 style AR-15 butts, you can get these extenders.
This is the Bell & Carlson model

There are also AR15
AR15 rifle cheek "swells" a simple solution that takes minutes to
mount.