Emwonae, your question is not a new one, here's basically the same message thread from a few weeks ago:
http://thefiringline.com/forums/show...threadid=54985
As for Brookfield Precision mounts, they don't sit as low to the receiver as the ARMS #18, and are a bit spotty in production and availability. The last I heard, Brookfield wasn't even tooling up to make any more until probably this coming fall.
Smith Enterprises makes a pretty good facsimile of the Brookfield.
GG&G made a spot-on copy of the ARMS #18, until they received a court order to cease and desist for patent infringement against ARMS.
The ARMS #18 is still available, if you want to see what one looks like, check out the above message thread for my message and image URL, as well as going out to Fulton Armory's website and looking at their pictures. Fulton is now a stocking dealer of the ARMS mount, since Brookfield is so hard to come by.