Seems like the short 6mm - 6.8mm is getting a lot of attention these days. Flavor of the month sort of thing.
Here's a 6 that's made from a necked down and blown out Grendel :
6mm DTI
http://www.predatormastersforums.com/forums/ubbthreads.php?ubb=showflat&Number=1633906
It gets the 6mm pills moving pretty good.
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NO STEP ON SNEK