All I know is that H&H say their self-opening guns will open even if held upside down. On others which might be described as easy opening guns, some of which seem to just have weaker mechanisms of the same make as H&H, the gun will only "ease" open with assistance.

Try this if you get your hands on one of the Spanish guns: A. close and move the toplever to the right while holding the gun upright. B. Same as A but after firing. Some of these only pop open if not fired. I don't know why.