I recently zeroed my new Bushmaster upper and the front sight post is almost as tall as the front sight protective ears.  The upper is a 20" flat-top with an ARMS #40.  I had another upper (RRA receiver and Bushy 20" barrel) that zeroed fine with no elevation adjustments required.  It also had an ARMS #40.  What's up with all that?  

When I zeroed my 16" upper and the post came above the ears also.

Mine too, with detachable carry handle. This is odd. What knid of rear sight you have?

MN

Here's the reply I got from Bushmaster on the subject:

[b]Subject: Sight Heights


            When Colt came out with the A3 design with the detachable carry handle they made the front and rear sights .040" higher than the sights used on A1 and A2 rifles. The front sight forging is the same but the front sight post when set for mechanical zero is even with the top of the sight ears instead of below them as the plane for the sight post is milled higher. You will notice on 20" A3 barrels there are two raised bands on the barrel under the handguards. This designates that the barrel has the front sight base with the front sight that is milled high for the front sight post. Colt parts lists have a different part number for the higher front sight base barrels and sight bases for A3 and M4 barrels.  
            We decided when we began to build the A3 models with detachable carry handles that we would keep the sight heights the same for all models. Our A3 handle, Flat-top Rear Sight and Rear Flip Up Sight has the same rear sight aperture height as on the A1 and A2 receivers so that all barrels with standard height front sight bases will interchange. The Mark Brown Flat Top Mount also uses this same rear sight height.
            The A.R.M.S. Swan Sleeve and # 40 Flip Up Rear Sights have the higher rear sight aperture height. When used with standard height front sight bases the front sight post must be turned out to the .040" higher position where it is even with the top of the sight base ears for mechanical zero.
            We now carry a .040" taller front sight post http://www.bushmaster.com/shopping/scopes/9349056-m.asp , part number 9349056-M for $4.95, to correct this difference.

           Thank you.[/b]