Re: 300 Blackout Thompson G2
These have been around for the best part of thirty years. We began by using the 45 Winmag as it was a rimless cartridge with almost identical dimensionality to the .45apc, allowing us to run from subsonic to, well, 45 Winmag an awesome cartridge in its own right (40,000cup vs approx 12,000cup). They had their plusses and minuses, perhaps the only real plus was almost no mechanical noise, making subsonic an extremely quiet affair.
Downsides included generally funky reloading, accuracy was extremely load sensitive, cramped optics. Eventually it became pretty clear, one could derive generally the same results in the pistol configuration for the yards that it was capable of shooting. You wouldn't believe the number of barrel and chamber configuration tested.