You'll likely want a riser for the AR15. That said, when shooting with both eyes open (typical red dot technique), you should be able to superimpose the dot from one eye onto an image of the target coming from the other eye in your brain without any trouble.
I had one on my VZ58 for a while, it worked perfectly, then I sold the rifle for a handgun.
I also use a fast fire on my M14 for pushing bushes in deer season. I love it. It's a nice little optic. If you do go this route you'll be happy.