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Barska AR Riser Mount. YouTube has a video showing you how to do it.
I use a carry handle with an added picatinny on top of that for my scope, that way I can still use the iron sights for shorter ranges.
If you get the scope high enough to clear the front sight it is very ungainly to use. You are best off replacing the gas block (front sight) with a low profile one to use a scope. You could get an AR with an aluminum handguard with the picatinny rail and no front sight. Then put your scope on and use offset AR backup iron sights which sit at a 45 degree to the scope and are not in the way.
First off 6x is a bit weak for 3-400yrds, in my opinion. I have an 8x on mine, and past 300 gets tricky. That said, replace the gas block either way, much better than raising the scope that far from the barrel.