I've never used on or seen one up close, but people talk about that brand, Spikes, as if it is very good.
Wideners has one for a few dollars less that I have seen up close, and it seems very good - and has favorable reviews on another forum (M4Carbine.net) - the URL is
http://wideners.com/itemdetail.cfm?i...13&dir=700|863 - note I do not work for any of these companies, just want help you get your lower.