I know that this isn't exactly tech talk but when I find good deals I like to pass them along.
Brownells. com   Tango Down Arc Magazines for the AR-15
I bought 20!

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I believe this is what you are talking about.  http://www.brownells.com/.aspx/pid=27228/Product/AR-15-M16-ARC-MAGAZINE
 
Deals are deals, tech or not.  I happen to usually browse tech deals sites so that is what I post here.