Well seems the AR's started in the 1:24 twist rate, but more options are becoming available. I see the bolt guns running 1:16 twist. Figured Id start a thread with what Im finding is available, so others joining the game will have some options. Ill try to update as I go.
Mcgowen will do 1:10 thru 1:24 http://mcgowenbarrel.com/chrome-moly-barrels/
1:14 twist
http://kmtactical.net/450-bushmaster-barrel-bolt-combo.html
1:16 twist
1:18 twist
http://columbiariverarms.com/product/450b-20-1x18-standard-mid-1116-machine/
https://ritchsprecisionguns.com/product/450-bushmaster-caliber-20-length-barrel/
1:20 twist
http://shop.saternbarrels.net/products/16-450-bushmaster
1:24 twist
http://www.tromix.com/450-bushmaster-parts.html
http://shop.gryphontradingllc.com/450-Bushmaster-165-Barrel-Bolt-Combo-416R-Stainless-Threaded-450BBC750TM16.htm