Unfortunately you need to remove the barrel nut. There is nothing else holding the muzzle nut in place. You probably just need to "crack" it loose. Stick the gun in a vice so you then have both hands on the circlip pliers. Alternatively, you could make a tool from a hex socket. Grind / cut it to make some (maybe two) pins to fit the muzzle nut. Then you can fit a larger bar for more leverage. Good luck!