Hello, I just purchased a Leupold 3x9x40 scope. I have a Colt 20"A3 flattop with a removeable handle. What do I need to put the scope onto the flattop? Thanks

I assume you mean a 3.5-10x[b]40[/b] scope.  Is that right?

This weekend I put a Leupold Vari-X II 3-9x40 on an A3 upper.  I used an ARMS #36 Swan Sleeve rail and a set of ARMS 22L quick-detach rings.  I also needed the ARMS spacers, which let you mount the 1” scope tube in the 30mm rings.  I bought the ARMS stuff from a friend (thanks, zzxx99!), but here are some links to the pieces mentioned.

[url]http://www.mountsplus.com/products/ARMS/arms-22L.htm[/url]
[url]http://www.mountsplus.com/products/ARMS/arms-37.htm[/url]
[url]http://www.mountsplus.com/products/ARMS/ARMS-36s-ex.htm[/url]

The picture of the #36 is actually wrong.  The front of the rail is slightly different.

The issue with Leupold and some other scopes is their long eye relief.  The scope must be mounted far enough forward that you can’t mount it directly to the receiver’s flattop.  I’m very happy with the setup though.  

If you want, I’ll post some pictures tonight.

I tried to use a 3.5-10x40 using the mount Troy suggests, and the eye relief was too short for me.  Leupolds have very long eye relief.  If I do it again, I'll go with a SWAN extended sleeve.