I would have said the 6920, Spikes or a BCM yesterday. Today I'll say a used, beat to hell Olympic Arms or a non-functioning DPMS.
Seriously though, go with the 1/9 twist if you can. The 1/7 if you plan on feeding it a steady diet of 69+gr match loads. 1/9 will handle everything under that very well.
Seriously though, go with the 1/9 twist if you can. The 1/7 if you plan on feeding it a steady diet of 69+gr match loads. 1/9 will handle everything under that very well.