I have a Bushmaster with 20" barrell and A3 sights. Is there any flat top scope mounts out there that will give me enough height to get a  unobstructive view with a 3-9X40 scope with out the front sight getting in the way of the view? Does anybody have this mount on there's?

http://www.johnmasen.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=J&Product_Code=1552&Category_Code=XTSM

Get the ARMS #36ex (OSR)optical sight rail, med. ht, rings. Anything 5 power up in magnification on a 20" barrel will not be seen due to it being out of the optical focal plain.
Good shootin, Jack  

If you get the scope high enough to get it over the irons, it will mess up your cheek weld.  Like Jack says, you will not even see the front sight at 5x when the scope is at the same level as the irons; at 3x the front sight will just be blur and it will not significantly interfere with your sight picture.

The bigger problem is eye relief.  This can be solved by getting an extended rail (there are a few different kinds) and some 1" high rings.