Bottle won’t unscrew from gun

I have a .25 cal. Hatsan bully. I bought a new huma air regulator. When I went to install the regulator I degassed the gun and dry fired it. I can’t unscrew the bottle. Won’t even budge. Used a rubber band wrench I’m afraid I could damage the gun if I force it any further. Does anyone have any suggestions thanks for the help.
 
Is the bottle supposed to be removable from a Hatson Neutron Star? I read a review from someone that complained the bottle was not removable.

The OP has a Bully. According to this tear down video of a Neutron Star the bottle screws off once all the air is out of the gun, just like the Bully.

 
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