Those B-Square adjustables are worse than the C-mounts. The first and only one I had was on a Diana 54, and after a couple hundred shots one of the caps split and broke right off. Not to mention that it has a confusing array of little adjustment screws that never seem to work or stay tight. Add to that the fact that they don't even seem to be made of steel, some kind of cheap alloy rather, and it all adds up to junk. The 5039 is your best bet. All of my high power air rifles have them, and so far none has had a single problem or gone out of adjustment.