You will likely find you will not be able to unscrew the bottle and nor should you ever attempt to while the rifle is holding pressure, all Daystate bottles are designed to be fixed and do not have a valve in the end like theoben bottles etc. There is a proper bleed tool for clipping onto the foster fitting that will open the fill valve to empty the gun, that's the official method. If you have not got the correct tools then shoot it empty, dont be tempted by methods such as backing off the air gauge, dangerous as well as potentially giving the new owner an issue.