Accessories Rail on top of receiver

[Generator]

With my Big5 2-barrel in 10 day jail, i've been looking all over for accessories and stuff. I came across this accessory rail that seems to sit on top of the receiver. What I like is that it doesn't affect swapping out the barrels.

Does anyone have any experience with this, or anything like it?

http://www.amazon.com/Mossberg-Maverick-12Ga-Shotgun-Scope-Mount/dp/B001UK5AXG/ref=sr_1_33?s=sporting-goods&ie=UTF8&qid=1291313497&sr=1-33