I have seen small (.6 or so) diameter front sight/gas blocks on some old M-16A1 uppers. I have been thinking of making an ultra lightweight free floated 20" upper with a skinny custom turned barrel. It would use A-2 sights, so would those very small A-1 gas block sights still work with A-2 uppers?  

What diameter are they and can you can a modern H-bar turned down at the gas block to work with one of these gas blocks or would you have to start with an A-1 barrel?

Where could i get those gas blocks?