Well, No your really not past the point of dimenishing returnds past 20" unless you are doing building searches with it...for small varmets at 200 yards they are quite popular and availible.(it just caters to a different crowd than you or me for that matter as I have an M4 with an EOtech)
some examples of the types of AR rigs that use such a barrel(22",24",26").
http://www.accuracysystemsinc.com/ar15_conversion.html
some examples of the types of AR rigs that use such a barrel(22",24",26").
http://www.accuracysystemsinc.com/ar15_conversion.html