I think you misread their information:
The Super 42 is for use with Carbine length buffer tubes and is not compatible with rifle length or A5 buffer tubes/receiver extensions.
We do not recommend the Super 42 with 300 Blackout subsonic ammunition. The Super 42 Spring may be too powerful and not allow the rifle to function properly.
As long as your lower has a carbine (or pistol) extension, it should work.
Patriotism means to stand by the country. It does not mean to stand by the President... - Theodore Roosevelt, Lincoln and Free Speech, Metropolitan Magazine, Volume 47, Number 6, May 1918.
Every Communist must grasp the truth. Political power grows out of the barrel of a gun. Our principle is that the Party commands the gun, and the gun must never be allowed to command the Party Mao Zedong, 6 November, 1938 - speech to the Communist Patry of China's sixth Central Committee
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