I have a Colt 3x20 AR-15 scope with handle mount for my Colt SP1 and was thinking about mounting it to my pre-ban M4 wannabe.

Anyone done this?
Is there a decent mount/rings for it?

The handle mount is removeable from the scope so normal rings could be used to attach it to the picatinny rail.
I have an ARMS #40 rear sight, so I hope they don't interfere. If they do, so be it.

Have a look at the ARMS #19 ACOG which is meant to mount a compact ACOG with a single screw to a flat-top rifle. This guy is quick detachable with a pair of levers - there's a cheaper ARMS mount with thumb screws.

If you want to remove the mount from the scope what is the outside diameter of the scope? These guys are pretty small so I'm thinking 1". There's pleny of quick detachable rings for 30 mm scopes and some for 1" ones as well.

Again I like the ARMS #22 rings which come in three heights to suit most any need. The "M" medium models feel about the same height as the stock iron sights giving me about the same cheek weld.