JBM shows marginal stability with a 105 Amax or a 105 Berger from an 8 twist at subsonic speeds.
If you were to drop down much in bullet weight to gain stability, you wouldn't be gaining much over a subsonic 223, which when run through JBM with a 70 grain Berger and an 8 twist shows great stability, though much less so with longer bullets. It appears the heavies designed to fit in AR mags will stabilize, at least according to this program.
If I was looking to throw a big subsonic bullet, I'd want to go a lot bigger than a 6mm. Otherwise I'd just run a 223 (or a 22 LR, for that matter) and be merry. Dump the 6mm barrel and find an 8 twist 30 cal....AAC makes ready to run 300 blackout barrels chambered and threaded for Remington 700s, sold through Midway.
Linky Poo 
