ASR outer barrel:Cutting and Re-threading

Hi everyone, my current project for my ASR is to cut down the barrel approximately 6 inches so it can use a 300mm inner barrel / re-thread the outer so the end cap will screw on properly.

I was wondering if there was anyone out there that might know the correct tap to thread the outer barrel of the asr. After measuring I can say that the correct tap is between m20-m23 but I can't find a conclusion yet.