I got a call from my friends at Stillwood ammunition a few days ago after they were informed by a customer they had experienced multiple destructive baffle strikes from ammunition they loaded using lead free 125gr slotted copper Precision Component Technology bullet. The customer was shooting a new SIG Sauer MCX pistol in 300 Blackout. Josh, from Stillwood, and I brainstormed on what the cause could be, as I have shot literally hundreds of rounds of that ammunition through several platforms and several silencers without any issue. After going through everything I could think of I asked Josh what the twist rate was in that MCX pistol. He told me his customer informed him the twist rate was one in six, and I was sure that was wrong. Josh agreed that seemed awful fast so I took to the SIG website and could not initially find that information. So I searched for the Owner’s Manual for the pistol to find answers and sure enough it states that every model of the MCX has one in six twist rate.
In my opinion, and the opinion of Stillwood and Barnes Precision Machine this is an entirely way too fast twist rate for supersonic 300 Blackout. I called tech support at a well know ammunition manufacturer that I have a relationship with and learned that with such as twist rate the bullet is turning 300,000 rpm. I was also told that even with the .22 varmint bullets designed for a fast one in seven twist at extreme velocities, such as the .220 Swift develops, the bullets will come apart before you reach 290,000 rpm. So we know 290,000 is beyond redline for varmint bullets designed for fast twist rate and velocities. He proceeds to tell me with that fast twist rate in a 300 Blackout the lightest bullet that could be shot would be 220gr at only subsonic velocities. This information should be easier to locate.
So, it appears that light supersonic bullets can and will come apart due to centrifugal forces from twist rates of one and six in the SIG MCX 300 Blackout barrel and could cause damage to silencers. You need to exercise extreme caution when doing such. Stillwood will be adding such a disclosure to their website. With all that in mind, a one in six twist rated would be optimum for 300 Blackout 220gr or heavier subsonic ammunition.