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A riser that is 1" will let you co-witness with the iron sights in the lower 1/3 of the red dot sight. A .83" riser will get you absolute co-witness with the irons in the center of the RDS. I prefer lower 1/3 as it gives you more field of view unobstructed.
Some popular RDS are the Vortex Sparc and Strikefire, Primary Arms Micro Dot, Bushnell TRS-25, several others, and of course, you can always pony up and go EOtech or Aimpoint for a military-grade sight, but it will cost you more than the rifle.
Unlike a scope, remember not to look into the sight, just look for the red dot, both eyes open. That's the secret to shooting with a RDS.