I have this, but it isn't what you're looking for.

[img]albums.photopoint.com/j/View?u=1130110&a=10062068&p=38308723[/img]

So, let's see...  how about this from the Bushmaster FAQ?

[img]www.bushmaster.com/images/upper.gif[/img]

If you're talking an A2 windage knob, it's a roll pin.  Use a punch (Bushmater & Brownells have a 4-piece set that covers the sizes for ARs), drive the pin out just far enough to remove the knob.  Be careful, there's a spring-loaded BB that will fall out (with the spring).

-Troy