Parralax error occurs when the optics are not focused on the target at the distance you are shooting. Moving your head will cause POI to change. These optics are parralax free from 50 yds and beyond, so inside 50 yards they are not perfectly focused (parralax free) and you will see slight POI shift when you move your head in relation to the optic from shot to shot. Shooting 50 yds or greater there will be no parralax error. Make sense? The strikefire has less than 1" error inside 50 yds which is nothing to worry about with an AR unless your bench shooting, but then you should be using a scope for that. I had a strikefire and did not like it. It wasnt poor quality but the Bushnell TRS-25 beats it in every way for $130. The glass has alot of distortion towards the edges givng a kind of fishbowl effect, resolution in low light was poor which made contrasting between the overly bright dot and poorly defined background a blurry mess. The brightness does not go low enough for shooting in the evening. The TRS has better glass, a slightly smaller dot and better brightness settings to work in any light. Consider its half the price of the sparc, mounts lower and doesnt have a huge control panel on the left side blocking your field of view with both eyes open. The 2x doubler barely makes a difference other than narrowing your FOV right down to a dime-size hole and making optical clarity even worse. Just my 2 cents.
