CMMG Micro Gas Block .750"
We use CMMG Micro gas blocks because they're one of -- if not the -- lowest profile gas blocks on the market. This ensures that they'll fit under rails like the URX.
We also find that the inside diameter is usually a little tigther than most gas blocks allowing for a tight gas seal. At the moment, this is our favorite gas block.
Only one caveat: It requires a 5/64" hex driver intead of the standard 3/32"