just watched a video showing the 308 to 223 coversion. appears to be a pattened block that's attached to the 223 upper. I would primarly want it for the 308 but it would only make sense to have plans for another upper. pattened block means sold serperately and $$$ I bet. and technically it's a proprietary lower in the same way apple made the he iPod. only works with everything if you have the componentsto make it work.