I have a detachable handle A3 Bushmaster and when using high rings on the rail the scope would be obscured by the front sight. Am I missing something or is there a solution?

You'll find out that you will need Extra high rings. The bore line is so high on an AR-15 that with regular high rings you won't be able to scrunch down hard enough to be comfortable and see the scope.
The front site blocks some of the light coming in but not all of it. Unless you have a low magnification scope you barely notice it!.............DJ