I am looking at Ruger AR 556. It comes with iron sights. Problem is from what I've researched its a pain to change front sight/Gas Block because they have theirs mounted differently. Can you mount the scope to were the front iron sight won't be seen in scope?
You won't even be able to tell the front sight is there through the scope if you go with a 3-9 power like most have suggested. With a 1-4 set to the lowest power you will see the front sight.
It is relatively simple to change out the fixed front sight for a low profile gas block, but if you don't feel comfortable doing it yourself a gunsmith will do it in about 2 minutes for $20 or so (not including the price of the new gas block.)