One beautiful thing about the AR15' s is the modularity of parts and the rails that allow you to quickly reconfigure the rifle.
Here is the rear iron sight that can be removed in a few seconds without special tools:
I tried stacking the standard iron sight with a high-powered scope on my .22 cal. AR-15.
This worked OK, allowing me to use both sights, but I had to take my cheek off of the stock when using the scope.
Stacked telescope over iron sight on a .22 cal. AR15 Smith & Wesson
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, making perfect alignment tricky.
However, when I tried it on my .223 AR-15, the height difference was too much to effectively use the scope.
It's interesting, I've seen several people do this sort of sight stacking on AR's, and trust me, it's not ideal.
Don’t try stacking a .223 iron and telescopic sight
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 anfd adding a cheek riser so that I don't loose "cheek weld" on long distance sniping.
A butt riser allows for cheek weld on stacked sights
Using the Bell & Carlson A4 cheek riser is perfect for stacking sights!
The cheek riser is hinged, so that it rotates down, allowing you to use your iron sights with a .223 military style assault rifle with fixed front sights.
For the .22 caliber AR-15 models, they have a quick release front sight so that the front sight does not interefere with your red dot of telescopic sight.
However, you still have option of using a stock butt check riser, for an extra high stock for the non military AR's:
A plastic high-rise stock butt for the AR15
This make it super-fast to change sights, so I can use both iron sights and telescopic sights at will, depending on the distance of the enemy varmit.
See my full notes here on Co-witnessing vs. stacking of AR15 sights.