The PTS MKM Barrel Nut Tool is a specialized adapter that fits on the Mega Arms MKM licensed GBB from PTS. This tool allows you to use a large crescent wrench or channel locks to remove the barrel nut and access the hop-up in the rifle.
Without the MKM barrel nut adapter, you're not going to be able to remove the barrel and barrel nut. In the case of the LM4 powered PTS MKM, this is a required step to service or replace the hop-up unit or bucking. Anyone that works on their own airsoft guns and owns a Mega Arms GBB needs one of these.