Thunder Beast 30p-1 threads are :5/8-24 standard, also availible in M18x1.5, M18x1.0
For the standard 5/8x24 you need an OD of about .625 if the bore was concentric, most likey it will not be perfect so you may need alittle more then that. You will need to measure the OD of the barrels muzzel and see if you have room to turn the barrel down and thread it.