I apologize for the late reply.
If you are talking about a standard front sight that has been cut down then yes it will fit over it. If you are talking about a gas block that has the rail on top then no. It has to have a low profile gas block installed. This isn't a 2 piece drop in rail, you would have to remove the gas block to install this anyway. If you are already removing it might as well replace it with a low profile.
If your gas block looks like the one below then yes it will fit over it. You might have to take a hacksaw to the boyo lug and sling loop, but again you will have to remove it to get the barrel nut over the barrel.
![Image](http://jbfirearmsales.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/b/u/bushmasterorc223.jpg)