most red dot sights have parallax, do an easy test, lay your sight on a sturdy table turn it on and look thru it. line up the red dot with a distant target, now, move your head about so it appears as if the red dot is moving around with in the sight. if the red dot moves off of your initial target, your experiancing parallax. therefore, in order for your sight to be accurate 100% of the time you must always center the red dot within the sight. Aimpoint red dots are parallax free, doing the same test above with an aimpoint, the red dot always stays on the initial target, no centering of the red dot is required, whatever the dot is on you will shoot regardless of where the dot is in the sight tube. the aimpoints are a little spendy $250-$500 but they are well worth it, i have an aimpoint 9000sc on my 870 slug gun and i love it, they are built like a tank. also the new circuity in the aimpoints allow for 54,000 hours run time on one battery NO LIE!! so basically you can leave it on for 4 years. check out their websight www.aimpoint.com