If you can't find a large rock and sledge hammer, the Channel Locks or Vise Grips would be your next option unless damage is to be avoided... at that time, go get a strap wrench and give that a try... If it is still not giving up... Get a heat gun or super good hair dryer and heat that area well to break any loc-tite loose etc...
If neither the gun or dryer is available, completely tear down the gun and put magtube/receiver assembly in the oven at 200 or so to get it hot...
Brent