Just to play devil's advocate. I've owned a $100 chinese ACOG knock off and it was incredibly impressive. It held zero for the couple years and few thousand rounds I put through it. I don't recall stretching it to 600 but it was out to 400 on paper and steel often. I was always super impressed and actually regret selling it. Would be awesome to have on my rimfire AR plinker.their a difference in kit's & building firearms from scratch. I find popp'n a coyote in the head @ 600 is hard to believe all day who using a Chinese ACOG knock off. lol
It's all about what you want. For plinking at targets on the weekend why not use the $100 knock off. Will work perfect for most people.
If there are people shooting back at you, get the real thing. Your odds of it not failing are better.
That being said, I own real ACOG only at the moment but if i had a knock off laying around I'd have no problem using it in many applications.