I hadda AA S510 a few years ago and didn't have much trouble removing the barrel.
Try this reply:
You need to have a set of metric sized Allen wrenches.
Degas the air chamber by shooting it down until a dissipating hiss is heard.
Unscrew then take off the fill adapter protective cover.
Remove the barrel band by loosening the 3 set screws then slide it off of the shroud.
Detach the shroud by un-threading it from the barrel (rubber gloves come in handy for this part), then slide it off of the barrel.
Then loosen the 2 front set screws on top of the receiver. The barrel will then slide out of the receiver (heat may or may not be required).
Sometimes the brass piece will remain attached to the barrel, sometimes not. If not, you can cut small pieces of 1/4" wooden dowel rod and push the brass piece out the front of the receiver using the bolt's pellet probe to push the sections of dowel rod against the brass piece.
While it is out you can un-thread the two sections of the brass apart and inspect the breech O ring for its condition and replace it if necessary. The .22 caliber breech O ring is size 009.
When you push the brass piece back into the receiver with the barrel upon reassembly, be sure that the hole for the fill port is aligned exactly into proper position (this should be indicated by the marks left from the Allen grub screws). Otherwise, the adjustable velocity screw will not work properly, and your rifle won't operate efficiently or at all if this hole is blocked through misalignment.
Make certain to align the grub screw marks when reassembling, or things won't be right.
OR.....Here is a video that shows the barrel removal at the very end of the video.
