At one point during our impromptu but fantastically informative 15-minute master class on suppressors and internal ballistics from the Suppressor Gnome himself, he got a mischievous expression on his face and suggested that we would be interested in the hardware being displayed in a low-key way at the back of his counter. He was excited to show us how the MCX upper, which does not require a standard AR buffer tube, was direct-fit compatible with conventional AR lowers.
SIG will be offering a complete MCX upper conversion kit in .300 Blackout with an integrated (not integral) pinned-and-welded SIG suppressor, coming in a 16.1 inches barrel length. This makes it a single tax stamp gun, reducing cost and making it legal in jurisdictions which permit suppressors but prohibit SBRs. Interestingly, the conversion kit also includes a SIG folding stock to mount in place of the buffer tube, something which normally costs at least $200 to retrofit to a conventional AR.
Hollister explained that the 300 Blackout round was designed to completely burn its powder charge in only 9 inches of barrel, compared to the 5.56 round which was designed and optimized for the 20-inch barrel of the M16A1. This means that the MCX integral suppressed upper, despite having a relatively short barrel behind the suppressor, will shoot cleanly and reliably at full-rated velocity with both subsonic and supersonic ammunition.
The SIG MCX integral suppressor complete upper kit is expected to be commercially available in the first half of 2019, with pricing to be announced.