I don’t know anything about the SA20, but the video I watched about it after reading your post said after removing the barrel nut and forend, and holding the bolt lever( don’t know what it’s really called) so the spring doesn't whack forward when the barrel release button is pushed, the girl said that as you slowly release the bolt lever to un tension the spring, sometimes on new guns, the lever will stop a little short and needs to be pushed all the way forward to push the barrel free. Could it be something a simple as that? Hope you get it figured out and welcome to the site..