Sorry for joining the party late Gents, I've only just seen this thread, this is exactly why I always recommend stripping any new (to you) Airsporter or Mercury/Challenger and replacing the whole lot attached to the front of the piston (unless you know that it's been done already), even if the rifle is cocking and shooting fine, just strip it and see what's going on inside and normally the buffer washer is slowly choking up the insides and as James has found out they can be a right bitch to get the piston out, what you've done so far is normal practice, so don't think you've damaged anything, once it's all out, smooth off the cocking slot inside and out including the rounded hole, polish the piston up and replace the buffer washer and head and use a Meteor O ring as they are slightly smaller, TR Robb does a good head replacement kit with different size O rings, then get intouch with Tinbum to get either a set of guides made up (if your original spring is in good nick) or get a full kit made up as Nick uses HW springs and they are the closest springs you will get to original without having to go Titan, which are very good but will need shortening to fit in the rifle, before reassembling just have a little look at the nut that holds the threaded cup onto the back block isn't loose as if they come loose, then the whole back block will spin and not undo from the cylinder, you will need a very thin walled socket to do it up if it is loose but it's rare that it does come loose but I've had a couple over the years, when putting the piston back in take care not to damage the O ring on the threads that hold the back block in and the cocking slot that you should have smoothed out nicely, I've found putting the back block into a padded vice and pushing the cylinder onto the back block and getting started on the thread that way.

Pete